How to use apex:actionFunction and apex:actionStatus in visualforce page ?
A component that displays the status of an AJAX update request. An AJAX request can either be in progress or complete." 
A snippet of code for the Salesforce Visualforce Page :
A snippet of code for the Salesforce Apex Controller :
Here are some screenshots which are showing our apex:actionFunction and apex: actionStatus. We have embedded our Visualforce Page in a section on an opportunity. The page will look like below screenshot :
On click of 'Update Opportunity' button, an apex action will call that update the opportunity.
Here is a video that shows the working of apexfunction :
Look into the video before the click of 'Update Opportunity' button the opportunity name was 'Test opportunity' and after a click of the button the name update to 'Update Name'.