UV mapping; UV unwrapping
The map that decides which part of a flat image lands where on a 3D surface. Unwrapping is peeling the model flat to make that map.
See it
What it is
Every vertex carries a second pair of coordinates, U and V, that point into a flat texture image (0,0 in one corner, 1,1 in the other). That mapping is what tells the renderer which pixel of the image lands on which spot of the surface. Unwrapping is the act of making it: you cut seams and peel the mesh flat, like the papercraft net of a cube or the shape an orange peel makes when you flatten it. Blender's Smart UV Project does it automatically, marked seams plus Unwrap does it deliberately, and film pipelines spread the result across multiple UDIM tiles.
You need it any time a texture has to land somewhere specific: a label on a bottle, a logo on a shoe, baked ambient occlusion, painted wear on edges. You can skip it for procedural, triplanar, or matcap materials, which invent their coordinates on the fly.
Gotchas come in pairs. Texel density: if one UV island is huge and another is tiny, the same texture reads crisp on one part and mushy on the next, so scale islands relative to their real-world size. And seams are visible, both as a break in painted detail and as bleed at low mip levels, so hide seams where a viewer will not look (inside arms, under the base) and leave a few pixels of padding around every island.
Ask AI for it
UV unwrap this mesh in Blender for texturing: mark seams along hidden edges (inside surfaces, the underside, the back), unwrap, then pack the islands with consistent texel density for a 2048px map and at least 8px of padding between islands so mipmaps do not bleed across seams. Keep islands axis-aligned where the surface is flat, avoid overlapping UVs since I will bake lighting into this map later, and export as glTF with the UV set intact. Show me a checker-texture preview so I can spot stretching.