[UPDATE: As of October 2018, entities (both NPCs and players) can still targetlock any entity jamming them. If you are the ECM pilot, then while you have a successful jam going on an enemy, that enemy can targetlock only you and not anyone else. If you and 2 fleetmates are jamming the same enemy, then that enemy can targetlock all three of you, but not anyone else.]

An explanation to new players about how ECM jamming works.