Basically a line trace checks for actors in it’s path by “Colliding” with them. You then have a Hit Result which returns that actor or actors. You can then put that Hit Result into a Cast node and call functions inside that actor. So bullet shoots in a line, line trace gets actors in path, and executes your “Shot by Bullet” Event which could Destroy Actor.
The channel is something you can set per actor or actor class so that your line trace hits 1 actor but not another. I believe you can set channels in your project settings.