Hi Sanu,

In Salesforce we can achive AJAX request using the following component:
1) ActionFunction
2) ActionSupport
3) ActionPoller
4) RemoteAction

actionStatus visualforce component displays the status of an AJAX update request. An AJAX request can either be in progress or complete.
Depending upon the AJAX request status (whether AJAX request is in progress or complete), this component will display different message to user. In many scenarios AJAX request takes some time. So we should display some message to user that your request is in progress. Once request is complete, we can display some different message to user.

When we request for any AJAX using above component then if we want to show AJAX update status then we can use actionStatus component. Suppose if we are requesting any AJAX and want to show 'Processing...' text then we can achive this using actionStatus component.

Using actionstatus, we can also display some gif (graphic Interchange Format), which shows to user that their request is in progress. It gives very good presentation to end user.

You can learn more about here http://www.sfdcpoint.com/salesforce/actionstatus-visualforce-loading-image/
You can also check ActionStatus attributes https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_actionStatus.htm

Hope this will help you. Let me know if it helped or you need any more assistance. 

Please mark this is as the solution if it solved your purpose.

Thanks,
Sohan Raj Gupta