Overload
A piece is overloaded when it has to defend two things at once and cannot cover both after you attack one of them.
The usual pair is a hanging piece plus a mate square, or two hanging pieces. You take the first target; if the overloaded piece recaptures, the second target falls.
The rook on d8 defends the knight on d5 and also watches e8. Qxd5 asks the rook to choose. Recapturing leaves the back rank empty, and Re8 is mate.
When a single piece is the only guard of two important squares, it is a candidate for overload even if both squares look safe right now.