A partisan is someone who supports one part or party. Sometimes the support takes the form of military action, as when guerrilla fighters take on government forces. But partisan is actually most often used as an adjective, usually referring to support of a political party. so if you’re accused of being too partisan, or of practicing partisan politics, it means you’re mainly interested in boosting your own party and attacking the other one.
If something is prejudicial towards a particular point of view, you can call it partisan. You’ll often hear of the partisan politics in the U.S.— since politicians seem to be completely devoted to the agendas of their own parties.