What does breach chance mean?

Solution 1:

In FTL a Breach is a hole in the hull represented by a crack in the floor - you can see a hull breach in the room below the Engine room in the below screenshot

Breaches slowly vent oxygen into space and also must be repaired before the system contained in that room can be repaired and so they can be particularly nasty when trying to repair vital systems in a room.

The breach chance of a weapon is the chance that the wepon will cause a hull breach in that room when it hits (I'm not sure what probability "low" corresponds to)

Solution 2:

Breaching creates a breach in the hull. Breaches will slowly remove oxygen from any room exposed to the breach (without an airlock between it and the breach).