To break in is to force your way into a building to steal something. Somebody tried to break in while the family was away, and the police found a smashed window at the back.
It has other jobs too. To break in can mean to interrupt — sorry to break in, but the taxi's here — and to break in a horse is to train a young one to accept a saddle and a rider. You also break in new shoes by wearing them until they stop hurting. The noun is a break-in, with a hyphen: there was a break-in at the office last night. Compare break into, which needs an object: they broke into the house.
1phrasebreak inTo force your way into a building, usually to steal.
“Thieves tried to break in through the kitchen window.”
2phraseinterruptTo interrupt somebody who is speaking.
“I'm sorry to break in, but your taxi is waiting.”
3phrasehorseTo train a young horse to accept a saddle and a rider.
“It takes a patient rider to break in a young horse.”
4phraseshoesTo wear new shoes or boots until they become comfortable.
“Break in your walking boots before the trip, not on the first day.”