“Romex” is a brand name for a type of non-metallic (NM) insulated wire. It’s pretty much the standard for 95% of the wire that’s run in a typical house in North America, and kind of looks like a big flat extension cable. There’s an external plastic sheath that holds all the wires together (that’s the non-metallic part, as opposed to say, running it in metal conduit), and then each wire inside is also insulated, aside from the ground conductor. When you see something like 12/2 or 10/3, that’s the wire gauge (12 or 10 gauge) and then the number of current carrying conductors on the inside (2 or 3, plus a ground).