Position is where you are sat in relation to the button. Theres generally 5 stereotypical positions: Button, Blinds (SB,BB), Early Position, Middle Position and Late Position (also could have CO which is cut off, 1 before button). Your position plays a large role in what cards you should play, generally you can play looser in later positions as you will be LAST to act, which means you'll have more information then anyone else that round. In early position generally keep to your premium hands as you dont know what the people behind you may have ect.
Hope this helps