Pour overs are a specific type of hole that are formed by water dropping steeply over a rock, ledge or obstruction in the river. Pour overs are distinctive in that the steep drop of water leading into them creates a hole directly behind or even below the obstruction that can be extremely sticky and retentive and difficult to surf. Some pour overs can be extremely dangerous and may hold a swimmer. Where as others can be perfect for freestyle and great fun to play in. It is worth finding out from other paddlers if a pour over is safe or not before you paddle into / over them.
As with most river features if you are unsure if it is safe or not, leave them alone.