The tomato timer (also known as pomodoro timer) alternates between two modes: 

You can customize the length of the work and break interval and specify how many times you want the interval pairs to repeat on the right. 

Once the entire session is over you can take a longer break and create another pomodoro cycle once you are ready to work again.

During the work mode you will see a red tomato at the bottom right of your screen that slowly gets eaten. 

At the top you can see how much time remains in the session.
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Once the time is up you will hear a sound and the tomato will switch to a green tomato indicating that it's break time. 

Tomato timers for "work" (red) and "break" (green)

You can add a tomato timer to a task by typing how many minutes you want to work on it, how long you want breaks to be, and how many times you want to repeat it.

Clicking on this text/link will start the timer, and if you have the time tracking strategy enabled, it will also start time tracking that task. Pausing the timer will also pause the time tracking, and vice versa.

Browser notifications appear when each mode finishes.

You can pick the sound you want when the timer is up in your settings.