What makes Japanese selvedge denim different
Selvedge denim takes its name from the self finished edge woven right into the fabric. It's a tightly bound strip that stops the material from fraying, and you can usually spot it as a small coloured line when you cuff the hem. This fabric is woven on old style shuttle looms, machines that produce narrower bolts of cloth much more slowly than the mills used for most modern jeans.
That slowness is actually the whole point. Shuttle looms run at a fraction of the speed of modern looms, which gives the cloth a denser, slightly irregular weave with far more character than mass produced denim. Very few looms still run this way, and the ones that do are mostly kept running in small Japanese mills. That's part of why genuine selvedge denim is naturally limited, and why every metre carries something of the weaver's hand in it.
For the person wearing it, all of this adds up to fabric that's heavier, holds its shape well, and fades unevenly over years of wear. Those fades become unique to whoever's wearing them. Combined with how durable and repairable the fabric is, this is why selvedge denim is widely seen as the benchmark for jeans built to last for years rather than a season or two.