A timeout happens when a program waits for a result for a set amount of time and then stops waiting. For example, a page may show a timeout if a server does not reply soon enough.
Imagine calling someone and hanging up after waiting several minutes. You know you did not get an answer in time, but you do not know whether they received your message.
A timed-out request may still have reached the service and finished later. Before repeating an action that changes data, the app may need to check whether it already happened.