Patching is the practice of covering one eye to be able to exercise the other eye independently. If one eye is improving faster than the other, or if you started with a large gap in refractive state between the two eyes, you might patch the good eye and try active focus with just the bad eye for short periods. This technique is not recommended when starting out with EM methods. This is an advanced technique to try after several sphere reductions.
Patching is also used in conventional optometry for vision training and for rest while an eye is healing from trauma.