Some inexpensive dimmers or other lighting accessories have AC receptacles that accept only 2- prong plugs.
Some older residences do not have 3- prong AC power sockets in the walls.

This heavy-duty 3 to 2 Prong, Grounded AC Plug Adapter solves the problem of plugging in 3- prong fixtures or appliances into sockets that accept only 2 prongs.
This is a pack of 2 adapters.