If the hole is below and to the right of the oil pump, it is a plug for the oil passage between the pickup tube and the oil pump. If open, the oil pump will not pick up oil, hence no lubrication to your engine. It's a friction-fit plug, slightly domed so when driven in and flattened on the top, it creates a tight fit. If this is a new/rebuilt engine, someone bungled the installation of the plug. Most people will tap a pipe thread in these holes and use threaded plugs, instead. Much more reliable.
Frank Miller