I was wondering what the lores are good for, like common lore or forbidden lores. I dont understand what they do

They each represent knowledge about a certain subject. If your character wanted to know whether there's a psyker power which would create the specific patterns found at a murder site, Forbidden Lore (Warp) or Forbidden Lore (Psykers) would grant you that knowledge.

oh ok thank you, how would i use them if i was playing a game. And one more question how does threshold for phychic powers work.

oh ok thank you, how would i use them if i was playing a game.

You use them the same way as any other skill: You declare you try to perform some kind of action (in this case, usually remembering something about a certain subject). Then your GM tells you to roll on the skill with a certain bonus or penalty, depending on how easy to remember the information is. If, for example, you'd like to know whether there's any psychic power that could have slain someone the way he was slain, you ask your GM "Do I know any psychic power that could have done that?", whereupon your GM will likely say "Roll Forbidden Lore (Psykers)" and give you the information you're looking for if you make the roll.

And one more question how does threshold for phychic powers work.

The way they are described on page 161 of the rulebook.

oh gotcha ive seen people use that on youtube

thanks for the help so it is pretty good to have