'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.