It measures the strength of a feeling you have, like hunger, thirst, the need to eat sweets, the need to smoke a cigarette or the need to drink alcohol.
You press the + and - buttons whenever the strength of the feeling changes. If the feeling strength raises above the alarm level, it plays a "Think" reminder sound.
You then have to choose if you want to give in to the feeling, or to withstand it.
This way you will actually think about giving in to a feeling, or if it is just a habit. It may help you to reduce giving in to habits.
The Reset (R) button is used to reset the feeling strength to a preset level. For example after you have given in to the feeling.