In chess there is a way to give a discovered check where the piece moving will give check and reveal a line of attack behind it Knights are good at this but bishops and rooks can also sometimes move to give double check.
fig 1a. Is this checkmate? Yes/No
fig 1b, I have removed blacks bishop on d6, is it still checkmate? Yes/No
fig 1c. I have removed just the knight and the bishop is back is this position checkmate? Yes/No
Well yes the first diagram is checkmate, the other two are not checkmate!!
When the king is in check from two pieces at the same time, because you can only block a lineof attack in one one move and you cannot block two lines of attack with any piece move from any square!! In figure 1b the knight gives check and can be captured in one move, and in fig 1c the bishop only gives check and can be captured or the line of attack can be blocked with the pawn moving one square forward. In figure 1a the bishop and the knight cannot be captured in one move, if it were allowed, the knight would be captured and the king is still in check from the bishop, or alternatively if the bishop is captured or blocked the knight still attacks the king.
This is why double check is so powerful. In figure 1a, we know that it is double check and the king must move that is a rule now! but the king is never allowed to capture it’s own sides pieces or pawns so all it’s squares are killed off by the pawn on h3 and g2 and g3 is attacked by the bishop and the king can only move one square in any direction, the rook attacks h1 and g1 flight squares from f1. The king is checkmated. There is no escape square or block or capture.
Alternatively if you take away the f1 rook it is no longer checkmate as the king has two flight squares g1 and h1.