The stamps don't have any fixed meanings, but here are some ideas for when to use them.
Happy face: when you worked hard on your diet or something good happened
Sad face: when you ate too much or something bad happened
Meal: when you overate, or when you held back
Beer: when you went out drinking, and so on
Toilet: when you had a bowel movement, or when you were constipated, and so on
Ball: when you exercised, and so on
Star: when you had your period, reached a goal, or marked another special occasion
There are no strict rules for any of the stamps, so feel free to use them in whatever way suits you.