Block in Chess

Diagram 1

White Queen threatens to checkmate after Qxh7. In order to prevent Black interposes a piece ( Bishop ) between the White Bishop and h7-pawn : Bf5 . This is called as block.

Diagram 2

After Re8 , Rook attacks the King and checks . You can block this check by moving the Bishop to e8.
1. Re8 Be8

This is called as block-check.