https://www.scirra.com/manual/104/solid
Solid behavior
The Solid behavior makes other behaviors react to the object as if it were an impassable obstacle. Objects with this behavior are referred to as Solids. It affects the following behaviors:
8 Direction, which is blocked by Solids
Bullet, which can optionally bounce off Solids
Car, which bounces off Solids
Line-of-sight, which by default has line-of-sight obstructed by Solids
Platform, which can land on Solids. Platform cannot jump on to solids from underneath - for this, use the Jump-thru behavior.
Pathfinding, which by default uses solids as path obstacles.