A Grimshaw is a device found in chess problems in which two pieces (usually a rook and bishop) arriving on a particular square mutually interfere with each other.
The theme can be understood by reference to the displayed example by A. G. Corrias (published in Good Companion, 1917).
It is this mutual interference between two black pieces on the one square (in this case, a rook and a bishop on b2) that constitutes a Grimshaw.
However, any bishop or rook move must unguard one of the squares of d5, d6, d7 or d8, allowing White to mate on d5, d6 or d7 with the queen, and d8 with the knight.
Each of Black's defences produces a Grimshaw interference which stops him from capturing White's mating piece.
It is a complete block (if White could pass his first move, then he could reply to every black move with a mate), and White's key, 1.Bb3, holds this block, making no threat, but putting Black in zugzwang.