Skewer
A skewer is a line attack through a valuable piece, so that when it steps aside a piece behind it can be taken.
It is the reverse of a pin. In a pin the cheaper piece stands in front. In a skewer the dearer piece stands in front and is forced to move, leaving the rear piece hanging.
Re1+ checks the king on e2. The king must step off the e-file, and the rook on e8 is then unprotected. Checks that slide through the king are the cleanest skewers.
Bishops and queens skewer on diagonals the same way. If your king and a rook share a line with an enemy slider, assume the skewer is real until you block the line.