'Lavage' means 'washing'. To lavage a joint means to wash out any blood, fluid or loose debris from inside the joint space.
Lavage is generally combined with arthroscopic debridement, where fronds of joint material or degenerative tissue is sucked out using a small 'hoover' - ie. both washing (rinsing) and sucking. 'Arthroscopic' means that this is done using keyhole surgery, where a thin 'scope' is pushed into the joint (arthro=joint) to visualise the structures.