This is when you complete a specified amount of reps/rounds as fast as possible and record the amount of time it took you to complete. For example - row 1000m for time would require you to row 1000 metres as fast as you possibly can.
You will need to manage your effort during a 'For Time' Metcon. You should finish feeling like you don't have any more reps in you. On the other hand, beware that if you start off all guns blazing, you may burn out too fast!
In the likely event that you need to catch your breath, take 3 or 5 deep breaths before resuming. On shorter 'For Time' workouts, Drink water BEFORE or AFTER you start so that you aren't wasting time.
Consider that the clock is ticking up and that you want to minimise the amount of time it takes to complete the work.
The clock starts as soon as the first rep is initiated, and stops as soon as the final rep is finished.
Here is an example of 2 rounds of
5 x burpees
5 x squats