Lees are the dead/dormant yeast and all the other gunk that settles to the bottom of the fermenter. Similar to trub with beer. If left to sit on the lees, off-flavors can develop. Also, using sorbate and kmeta are less effective if you are sitting on a bed of lees as there can be viable yeast still present. The more viable yeast in the wine, the less effective kmeta and sorbate will be. Which is why we generally wait until the wine has cleared to add them before backsweetening. I add kmeta with the first racking to protect against oxidation(and every other racking after that), but not sorbate until I'm ready to backsweeten and bottle(waiting to reclear if necessary).