In most automobiles, the oil pan---the reservoir holding the oil that lubricates the motor's internal components---is situated beneath and behind the engine. The typically unshielded position leaves it vulnerable to some road hazards and bottom knocks resulting from driver error or an accident.
Rocks and other debris thrown up from the road can damage a vehicle's oil pan. Driving over a curb or a parking barrier, encountering a deep pothole, or driving into a ditch can also result in damage.
Usually, oil pans are merely dented and dinged up. A look at most oil pans would likely reveal a few dents, cosmetic damage only---unless they are so big as to cause interference with other parts. If damage is bad enough to create a hole, you must repair or replace the oil pan before driving the vehicle.
In order to replace or repair the oil pan, you must remove it. This involves draining the oil, lifting the vehicle with a floor jack and then removing the bolts that hold the oil pan in place. The exact procedure depends on the make and model.
A common experience with automobiles old enough to have undergone many oil changes is an oil leak that results from worn-out threads on the drain plug. Replacement plugs and gaskets for most vehicles' oil pans are priced at only a few dollars and are available at retail auto parts stores.
