A UV layout is the pattern you would get by cutting a 3D surface along chosen seams and flattening it. Textures are applied through it.
Seams
Every cut is a seam, and a seam is a potential visible discontinuity. Placing them well means putting them where they will not be seen: under an arm, along a hard edge, behind a join, inside a hollow.
More seams means less distortion but more discontinuity. Fewer seams means the opposite. The craft is the trade.
Distortion
Flattening a curved surface always distorts it. A checker texture applied to the model makes it visible immediately — squares that stretch or compress show where the layout is fighting the geometry. Some distortion is unavoidable; large distortion means a seam is missing.
Texel density
How many texture pixels per unit of surface. Consistent density across a model means consistent sharpness. Inconsistent density means one part is crisp and another is blurry at the same texture resolution — one of the most visible signs of an amateur asset.
Across a whole project, consistent texel density between assets matters just as much.
Packing
Fitting the islands into the square efficiently. Wasted space is wasted resolution. Modern packers rotate and nest islands well; the constraint is padding.
Padding
Space between islands, so mip-mapping does not blend neighbours into each other. Too little and small mip levels show colour bleeding at the seams. This is one of the more common causes of "my texture has weird lines at distance".
Overlapping and mirroring
Deliberately stacking islands so they share texture space. Halves the texture cost for a symmetrical object, and prevents asymmetric detail and correct lightmap baking. Common for game props, avoided for hero assets.
A second channel for lightmaps
Baked lighting needs a non-overlapping layout of its own — see lightmaps. A model destined for baked lighting carries two layouts.