In K2 WorkSpace,what's the mean about setting the delegate property on action of a workflow instance ?

When you delegate a task you effectively choose to temporarily "redirect" the task to another user so they can action it on your behalf.  You can define delegation rules via the workspace, which is handy (although delegation rules may be centrally managed when you don't want users to delegate tasks to other users who don't have the business authority to action those tasks).

Just to add on.  The Out of Office functionality is making use of the delegate functionality.  Delegate is different from a Redirect as redirect means handing off the workitem completely to the other person whereas delegate means sharing the workitem with the person.  So both the original user and the person you are delegating the workitem too can see the same workitem on their worklist.