The bottom bracket is the enclosed part at the bottom of the frame through which both pedal cranks attach to each other.
More details would help a lot. Does it occur with every pedal stroke? Does it depend on how much torque (force) you are applying? If you grab both cranks and hold one still while trying to rotate the other in either direction, are they solidly attached to each other? Is there any play (looseness) where they connect through the bottom bracket?