Slip-on names shoes you put on by sliding your foot in. There is no lace, buckle, zip, or strap to fasten. You will see it as an adjective (slip-on shoes, slip-on loafers) and as a noun (a slip-on). The point is the easy on-and-off, not a special brand. Loafers are a common kind, but not every slip-on is a loafer: some are canvas shoes, some are mules, some are smart leather. They can be for going out, unlike a house slipper. Informal everyday English; shops and clothing talk use it all the time. Though actually people stretch it a little to other things you slide on with no fastening (slip-on covers), but the shoe sense is the one this word is for.