This page opens with the timer already set to 25 minutes. Press Start or hit the spacebar and it begins; when time is up the alarm rings and the disk blinks. You can always drag the disk to a different time.
25 minutes is a great length for single-tasking. Watching the disk shrink creates a gentle deadline effect, so the same minutes feel more productive.
25 minutes is the classic Pomodoro focus block. Switch to Pomodoro mode and breaks follow automatically.