A resistance fighter is someone who is a member of a resistance movement and who actively takes part in combating their enemy.
Some served in the Bajoran resistance, the Maquis, the Resistance or Damar's resistance movement where they fought Cardassians, the Federation, the Nazi's and the Dominion. (DS9 et al; ENT: "Storm Front")
Bajoran resistance fighters carried subdermal implants of tritonium isotopes. The idea was when a resistance fighter was captured these isotopes could be tracked to find the whereabouts of that person. (DS9: "Indiscretion")
When the Cardassians started to oppose the Dominion, Damar was told by Major Kira Nerys to stop to think like a soldier and to start thinking like a resistance fighter and to make use of what you have. (DS9: "When It Rains...")