Light Bridges are a very basic entity. They consist of a flat beam of solid light that the player can redirect through portals.
Light bridges have many uses. They can help the player pass over troubled waters, or be used to block enemies, turning them around. They can be triggered using the linking system to create doorways or floors that fall out from beneath the player. In fact, two Light Bridges side-by-side are a perfect fit for Small Mario.
The emitter itself is not solid, though the beam is. This means that the effect is only noticeable if the beam is off.