Hey welcome to the forum. It is the auxilary belt tensioner, the pully wheel has come off.
It should look like this. You will have to buy a new tensioner. Replace the entire part do not remove the pully wheel from the new tensioner assembly.
bought it instald it no worries lol now how do i make the belt go around it ,it must have a pulley that i can undo then put belt on and tighgten ? wich one ,im sorry im not a mechanic but im learning lol
It is easy now that the pully is fitted put a wrench on the pully bolt (15mm socket) You are not trying to undo the bolt, all your trying to do is leaver the pully arm on its spring. Push the arm down slip the belt over and let go.
That is it, the tensioner is automatic and moves to the correct tensions it is completely self adjusting.