A partisan is a strong supporter of a cause, party, group, or person.
Or an armed group of people formed secretly to fight against an occupying force. In your case you are discussing WW2, so you will be talking about the second definition.
Since you are talking about WW2, you are probably discussing a particular group of partisans, possibly: partisans operating in enemy-occupied Yugoslavia, Italy, and parts of eastern Europe in World War II.
