This is one of the many Area Effects in Super Dungeon Explore.

Attacks or models with Cross X effect targets in a cross shape X squares on the flat sides of the source.  For instance a large model with Cross 2 would hit the four squares on each side. 

As of version 1.5 of the rules, all effects require line of sight, so Cross only effects models in squares that the user has line of sight to.

Cross 1
Cross 2