Lesson
White to move. Main line: 1. Rh8+ Kd7 2. Rxa8
Rh8 checks along the eighth rank. The king must step off the rank — and the rook continues to a8 and wins the queen. The king itself shielded the queen from the attack; once forced aside, the shield is gone.
A skewer is a pin in reverse: the more valuable piece stands in front. Whenever king and queen share a rank, file or diagonal, look for the piece that can exploit the line.
Defending against a skewer means breaking the line before it forms. Whenever your king and queen share a rank, file or diagonal with an enemy rook or bishop pointing down it, move one of them or interpose — once the check arrives, the choice has already been made for you.
If Ke7: Rxa8