Why 25
It is the Pomodoro length, and the most common answer to "how long can I actually focus without checking something". If you regularly finish a 25-minute block wanting more, try the 50-minute timer; if you regularly stop early, this one is right.
The break
Five minutes, away from the screen. The next block is armed when the break ends and waits for you to press start, so nothing is counted while you are making coffee.