Is any way to say simple what is source models in Magento?

Yes, it is . I think the simplest way will be to explain: Magento Source Models List - its list of dropdown menus available in magento by default. This dropdowns can be used at the admin grid or at the system configuration.  Actually you can use it anywhere where you make form using magento widget forms.

 How to use source model at system.xml file?

If you look at the line 43 at the code example below you will see tag <source_model />  . This is the place where you can put model name from the magento source models list below. Please pay attention at the previous line 42 it says <frontend_type> select</frontend_type> . This line define that our field will be select field (dropdown field) .

 Why Is it useful?

When you building new admin options or extension you can save time and avoid building duplicate of existing source model. Once you read once magento source models list you at least will remember what it doing. And the next time you will need to make source model you can save your time by using something what already exist.

What the other fields mean in system.xml file?

At lines 39-49 field properties declared. Except source_model which you can make on your own or use one from the magento source models list which already exist, there are some properties of the system admin configuration field.  Here are some of them: <label/>   - This is label of your field <comment/> - comment which will appear under the field. <sort_order/> - order of the field in fieldset. <show_in_default/>, <show_in_website/>, <show_in_store> - Where this field will be shown. I.e. scope of the magento website. <frontend_type/> - This field will set what type of field will be visible. Here are some of the possible values: select, multiselect, label, text, textarea, time, image. <frontend_class/> - This can add additional custom class for your field. <validate/> - If field should be validated you can specify validation type here. <depends/> - Here you can specify dependable configuration fields in magento admin. and of course <source_model/> which may have values from the magento source models list below:

Here is Magento Source Models List.

Model	Description
adminhtml/system_config_source_admin_page	returns list of all the items in Magento Admin menu
adminhtml/system_config_source_allregion	returns list of all country regions grouped by country
adminhtml/system_config_source_catalog_timeFormat	returns time format (12h or 24h )
adminhtml/system_config_source_cms_page	returns list of CMS Pages
adminhtml/system_config_source_cms_wysiwyg_enabled	returns list of text editor options
adminhtml/system_config_source_country	returns list of all countries from directory module
adminhtml/system_config_source_cron_frequency	returns list of cron frequence ( Daily, Weekly, Monthly)
adminhtml/system_config_source_currency	returns list of currencies
adminhtml/system_config_source_customer_group	returns list of customer groups
adminhtml/system_config_source_date_short	returns list of available short date formats
adminhtml/system_config_source_design_robots	returns list of robots options (index, follow)
adminhtml/system_config_source_email_identity	returns list of email sending identities (General Contact, Sales Representative, etc)
adminhtml/system_config_source_email_method	returns list of email methods (Bcc, Separate Email)
adminhtml/system_config_source_email_template	returns list of transactional email templates
adminhtml/system_config_source_enabledisable	returns list of two options (“Enable” and “Disable”)
adminhtml/system_config_source_frequency	returns list of frequencies (Always, Hourly, Daily, Weekly, Monthly, Yearly, Never)
adminhtml/system_config_source_locale	returns list of locales
adminhtml/system_config_source_locale_country	returns list of localized country names
adminhtml/system_config_source_locale_currency_all	returns list of localized currency names
adminhtml/system_config_source_locale_timezone	returns list of localized timezone names
adminhtml/system_config_source_locale_weekdays	returns list of localized weekday names
adminhtml/system_config_source_notification_frequency	returns array of hours frequency ( used for cron)
adminhtml/system_config_source_notoptreq	returns list of three options for customer widget mandatory indetificator (“Not”, “Optional”, “Required”)
adminhtml/system_config_source_order_status	returns list of available order statuses except pending payment
adminhtml/system_config_source_payment_allmethods	returns list of all payment methods
adminhtml/system_config_source_payment_allspecificcountries	returns boolean values for payment ("All Allowed Countries", "Specific Countries")
adminhtml/system_config_source_price_scope	returns list of price scopes
adminhtml/system_config_source_shipping_allmethods	returns list of all shipping methods
adminhtml/system_config_source_shipping_allspecificcountries	returns boolean values for shipping ("All Allowed Countries", "Specific Countries")
adminhtml/system_config_source_shipping_taxclass	returns list of tax shipping classes
adminhtml/system_config_source_store	returns list of stores
adminhtml/system_config_source_tax_basedon	returns list of values which tax based on
adminhtml/system_config_source_yesno	return boolean options (“Yes”, “No”)
checkout/config_source_cart_summary	return list of display options at the cart