Explanation

Escape squares for the king are sometimes called "luft" (German for "air"). Without escape squares, the king has no "luft" and is smothered by the knight.

Examples

Black can deliver smothered mate with Nf2#.
The white king is blocked in by its own rook g1 and pawns h2 and f2.

Sometimes this can happen in the opening:

White can deliver smothered mate with Nd6#.
The black king is blocked in by its own queen d8, knight d7, bishop f8, and pawns e7 and f7.
Note that the e-pawn is pinned by White's queen and cannot capture the knight.

Pattern matching

This usually happens with the king in the corner, or on the edge of the board.

Related patterns

In double knight mate, two knights checkmate the king.
In dragonfly mate, a knight checkmates the king, and a bishop prevents the escape.