Selvedge means the stable finished edge created along both sides of a woven fabric during production.
Why it matters in practice
A selvedge prevents fraying and can reveal weaving information, but it is not automatically a mark of superior quality.
A useful check before buying
Use it to understand grain direction and construction; in finished clothing, verify that pattern pieces have been cut on the intended grain.
What to keep in mind
Selvedge is not a universal quality grade. The same technique or feature can be executed beautifully or poorly, and it can be appropriate in one garment but unnecessary in another.