To enter a building or car by force, typically to commit a crime. The burglars planned to break in through the back door.
To interrupt or disturb someone or something. She had to break in during the meeting to deliver an urgent message.
To train or accustom someone or something to a new situation or task. It took a few weeks to break in the new employees to the company's workflow.
To wear or use something new until it becomes comfortable. He needed to break in his new shoes before the marathon.