Switch plates are those rectangular covers around light switch and electrical outlets mounted on the wall. Either because of frequent use with dirty hands, or with infrequent use because of dust build-up, it is important to remember to wipe these down perhaps on an annual or semi-annual basis.
Spray some all purposes cleaner on a towel and wipe down the plate and the switch itself. Don’t spray the switch directly as there is still power to the switch itself. If your cover plates are really dirty, you can remove them by typically removing the screw that attaches it.
Once removed, you can wash the plates in soapy water. Make sure they are completely dry before re-attaching them. We couldn’t find a simple video for this, so we just put a great overall video from Howcast on cleaning the kitchen.