Basically we used immediate="true" in CommandButton & CommandLink, when we don’t want our validation rules to be fired during any server request.

It is a Boolean value that specifies whether the action associated with this component should happen immediately, without processing any validation rules associated with the fields on the page. If set to true, the action happens immediately and validation rules are skipped. If not specified, this value defaults to false.

We generally use it to make functionality of “Cancel” button or “Back” button, where we don’t want validation rule to get executed. If we don’t use immediate=true then on click of cancel button also, validation rules will get executed.

Here is the sample code:

<apex:commandlink action="{!cancel}" value="Cancel" styleclass="btn" id="btnCancel" immediate="true">
</apex:commandlink>