A break-in period is simply the first hundred or so rounds (some guns more, others less, others not at all) for all the parts in the gun to "mate" and function properly.
Typically revolvers don't have a break-in period as they usually can function 100% out of the box. Like I said above some semi-auto pistols really dont require this break-in period either and they function 100% right out of the box.
Many makes and many models will be subject to various minor FFT, or FTF issues during the first few hunderd rounds (the break-in period) so if you read on these forums about failures of that nature often times you will also see questions or statements about the break-in period of the gun.
Hope this helps.