Grids is what i call graphic intended for walls that allow to see trough holes in them and stop or allow bullets, projectiles, players, monsters, with rules that are different from those of a solid wall. Some examples:

A fence might stop actors but allow bullets and projectiles

A tent or carwash brushes might obstruct the view but allow everything to pass trough, 

A futuristic forcefield might be designed to allow players and monster to pass trough, stop energy base projectiles.
And maybe doesn't stop bullets because the author of a project decides aliens didn't conceive solid bullets when they designed said forcefield.

Some grids seem identical in the preview, but there are variations with different levels of consumption, 

This collections contains more detailed grids, The pratice of using different resolutions might be questionable, but when it is required to represent thin rods or bars, the workarounds in low res are simply uglier than a high res net.