To break in means to enter a place illegally by forcing your way through a door, window, or other barrier. When thieves break into a store at night, they might smash a window or pick a lock to get inside and steal things. A burglar who breaks into a house is committing a crime.

The phrase also means to interrupt someone who's speaking. If your friend is telling a story and you suddenly break in with your own comment, you've cut into their conversation without waiting for them to finish. Teachers might need to break in when a classroom discussion gets too heated or off-track.

Break in can also mean making something new more comfortable through use. New shoes often need to be broken in by wearing them repeatedly until the stiff leather softens and molds to your feet. A baseball player breaks in a new glove by oiling it, bending it, and using it in games until it becomes flexible and easy to close. When you get a new bicycle, the gears and brakes might feel stiff at first, but they break in after a few rides.

The phrase works slightly differently depending on context, but each meaning involves some kind of forcing entry, interrupting, or making something ready for use.