Tilt your head sideways and the floor and wall lines lean with it. A roll rotates the camera around the axis pointing through the lens.
Turn a roll-capable tripod head or keyframe rotation on the virtual camera's lens axis. Tilt the frame and scene together; use a floor or window edge to make the change visible when the horizon is absent.
A pan or tilt changes the viewing direction left-right or up-down; a roll turns the image sideways. Large rotations can disorient viewers, so use them for a clear reason.
When to use
Use it to unsettle the viewer or create a deliberately tilted point of view. Avoid it, or keep it slight, when level information matters.