The pelvic floor is a network of muscles which serves as the floor to the abdominal cavity. These are the muscles you would clench tightly if you had to go to the bathroom but there were no bathroom around.
When you inhale, your diaphragm presses down hard on the contents of the abdominal cavity. The viscera contained in the abdominal cavity (the stomach, intestines, etc.) flow down and out in a distinctive tide-like motion resulting in abdominal expansion in the front, sides and back. The viscera also flow downward, pressing down on the pelvic floor.
When the viscera press down on the pelvic floor, its downward arch deepens, if you allow it. This motion is rather like stepping onto a miniature exercise trampoline. Upon exhalation, the pelvic floor springs back up, as though stepping off the trampoline. The pelvic floor helps you exhale if you do not clench the muscles which comprise it.