What is a concussion? & how careful do I have to be?

Very. A concussion is a brain bruise. Loss of consciousness, confusion, vision changes, balance problems, thinking deficits, etc can follow from injury to the central nervous system. Some can be permanent. Be careful because a second impact is even more dangerous. Many nfl players are concerned about severe long term adverse effects. Boxers too. Be evaluated by a neurologist.
Mild brain injury. A concussion is a mild brain injury. You should get plenty of rest, physical and mental, until you symptoms have resolved. Avoid contact sports until then.