The 45 RPM converter is the little circle that fits around the central spindle of a record turntable that fits the hole in the middle of 45 RPM records. For historical reasons having to do with the way records were played in jukeboxes, 45 RPM records have a big hole that means it is difficult to center them correctly on a standard record player. The 45 RPM converter centers them for correct play.
When I first obtained my record player, and I was reading the documentation, and saw mention of the "45 RPM Converter", I was worried that it was some type of complicated proprietary USB gadget that I would have to order online, but nope, it is merely a piece of plastic that probably costs a nickel to make.