HD Minecraft skins: what they are and when they work
What a 128×128 or larger skin is, and why vanilla Minecraft may not show it.
Standard is 64×64
A normal Minecraft skin is a 64×64 PNG. An HD skin uses the same layout at double or quadruple that size — 128×128 or 256×256 — so each face of the model has four or sixteen times the detail.
Where they work
Vanilla Minecraft accepts 64×64 only. HD skins need a mod such as CustomSkinLoader or OptiFine, or a server that serves skins itself.
Downscaling
An HD skin can be shrunk to 64×64 in the editor, but fine detail is lost — a pattern drawn at 128 pixels across cannot survive being halved. Skins marked Standard on this site are already 64×64 and need nothing.