In more detail
Francesco Cirillo developed the method in the late 1980s and named it after a tomato-shaped kitchen timer (pomodoro is Italian for tomato). Each 25-minute block, a "pomodoro", is spent on one task; after four you take a longer break of 15 to 30 minutes, so a cycle of four is 115 minutes including the short breaks. Interruptions are noted down for later rather than acted on, and a pomodoro that is abandoned does not count.
Why it matters
Fixed blocks make it easier to start a dreaded task and harder to drift into email or messages, and counting pomodoros shows how long tasks really take. The numbers are a convention, not a rule: many people use 50 minutes of work with 10 minutes of rest for deep work. A timer makes the boundaries explicit; a countdown timer also serves cooking, exercise and presentations.
Example
Starting at 9:00, four pomodoros run 9:00–9:25, 9:30–9:55, 10:00–10:25 and 10:30–10:55. A 20-minute long break then ends at 11:15, when the next cycle begins.