The linear damping is the rate of decrease of the linear velocity over time. A moving body with no damping and no external forces will continue moving indefinitely, as is the case in space. A moving body with damping will gradually stop moving.
Damping is not the same as friction - they can be modelled together. However, only damping is provided by Box2D (and LÖVE).
damping | number | The value of the linear damping. |