Patching is the practice of covering one eye to be able to give stimulus to the other eye, independently. This is most often applied to activate the non dominant eye (also see ocular dominance). 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 to 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 spherical reductions.
Patching is also used in conventional optometry for vision training and for rest while an eye is healing from trauma.