Truing a wheel is the procedure to realign a bent wheel by tightening or loosening the spokes until the right balance is achieved.
Truing might be needed after the wheel got bent because of a bump, or because one or more of the spokes got replaced, and must be adjusted to fit the rest. Also, truing can be a temporary fix when a spoke breaks and compensation is needed to keep the wheel acceptably straight.
Truing requires a specific wrench of the shape and size of the nipples, and is best performed using a truing stand. It typically consist (in its basic form) in tightening by 1/4 to 1/2 turn the spokes on the opposite - laterally speaking - side of the bend) and loosening the ones on the side of the bend.