When is the OnIdle function called? how we can call it repeatedly? difference betn thread and onIdle() task ?

When the application is idle and there are no messages in the application message
queue.

Returning a value of TRUE will cause the OnIdle function to continue to be called as
long as the application remains idle.

An OnIdle task executes only when the application is idle and there are no messages
in the message queue. A thread executes independently of the rest of the application.