Fork and double attack are the most common ideas in chess. When you attack two pieces with one move. When you attack two pieces with a knight move, it is called a fork. The double attack is when you attack two pieces with a Queen, Rook or a Bishop. There is no general reason for it. So let’s dive straight to the positions.
Nc7+! An amazing move to make. This is called “The Royal Fork!”. Its called especially when it attacks more than 2 pieces at a time.