This is a nickname given to a girl with breasts which are very saggy. Often when a girl loses large quantities of weight the breasts lose volume, but the skin is still the same size. In this situation her breasts become a similar consistency to sacks, loosely filled with cottage cheese. Hence the nickname Cheese Tits.
"I got this girl home thinking she was slammin hot. Got to second base and realized she had Cheese Tits. gross.