The condition is that one of the following happened to you:

Your flight is delayed for more than 3 hours
Your flight is cancelled
Your flight is delayed for less than 3 hours, but you miss a connection flight and arrive to your final destination more than 3 hours late
You were denied boarding because of lack of vacant seats (overbooking)

Other conditions:

The flight must not be delayed due to weather or other extraordinary conditions.
The flight must have been commenced no later than 6 years ago.