An interpleader is a form of civil procedure
employed when two or more parties want something that is in the possession a third party, and that the third party is willing to give up. The person holding the property files a lawsuit
against the parties who want ownership, effectively asking the judge
to decide who's entitled to the property.
Say a person puts money in escrow to buy a house. The contract for the purchase is disputed, and both parties claim a right to the escrow money. The escrowee could file an interpleader against the buyer and the seller to have the problem sorted out in court.
Usually, a person files an interpleader because they don't want to make the wrong decision. In the above case, if the buyer was at fault but the escrowee returned the money to him anyway, the escrowee would probably get in very hot water with the seller and might lose their reputation, or even be held legally liable for the loss.
In most cases, a person who files an interpleader is entitled to have their attorney and court fees reimbursed by whichever defendant loses.