UV mapping
Assigning every point on a 3D model a coordinate in a 2D texture, so a flat image can be laid onto the curved surface.
A texture is a grid in two dimensions; a model lives in three. UV mapping bridges them by giving each vertex a pair of coordinates (u, v) in the texture. The letters u and v were chosen precisely so they don't clash with x, y and z. To unwrap a model is to flatten its surface into a two-dimensional layout, rather like cutting open and unfolding a paper-covered object.
Those same coordinates then drive every texture laid on the surface — colour, normal maps, roughness — and they determine how filtering and mipmaps sample. Bad seams or uneven density show up immediately as stretched or blurry textures.