Any software project that has more than one developer working on it needs to have some way of ensuring that there is one and only one "official" version of the code. This is called source control, for which teams use tools like Microsoft SourceSafe and Rational ClearCase.
In the land of myth and legend, the code checked in to source control is ready to roll. Checking something in that doesn't work, or that prevents other parts of the software project from working, is called "breaking the build." On some teams breaking the build results in the offending developer working late, suffering humiliation from his peers, or having Vinny come by and break his knuckles.
Adhering to this discipline allows developers to join the team, get the latest copy of the code, and start working on it. Failing to adhere to this disicipline causes anguish, frustration, and despair.
That is all.