This may be caused by sweating in this area, especially if you have large breasts.
First, make sure you are wearing the right size bra. Too many women wear the wrong size. Go to a large department store where they will measure you. Don't be embarrassed. They do this for a living and see breasts every day.
Secondly, try to keep the area dry. Use talculm powder. Make sure the area is thoroughly dry after bathing before putting on clothes.
Thirdly, whenever you can, let the area see some sunshine. What I mean is letting this area of skin get some fresh air can work wonders. Even if it's only for half an hour at night before you go to sleep. "Pimples" between the breasts are easy to confuse with what may actually be a skin condition called ringworm, particularly if there are many of them and they're somewhat itchy. There are no actual worms involved, it's a fungal infection of the same variety as what causes diaper rash or athletes foot. It's not dangerous, only uncomfortable or embarrassing. An over the counter anti-fungal lotion will often help, but if the rash persists or spreads, a visit to the doctor for a prescription may be in order.