relations are collections of ways and/or nodes. hiking route relations are described here https://wiki.openstreetmap.org/wiki/Tag:route%3Dhiking . for most trails they are a a collection of ways (segments really) that make up a named trail. we don't use them consistently in the Sierra and less frequently in Yosemite since most trails don't have names. there are trails like the Mist Trail that have names and a corresponding relation https://www.openstreetmap.org/relation/6451906 or others like the Forsyth Trail that have names but are missing a route relation https://www.openstreetmap.org/way/131325321

generally speaking, they make it easy for apps and sites to find hiking trails and routes. sites like https://hiking.waymarkedtrails.org/#?map=12.0/38.0845/-119.5399 are purpose built for this.

when trails get really really long many tools start struggling to manage the size.. so now relations are capped to some number of members (like 2000-ish) but even that can get a bit unwieldy. so we break the big relations down into smaller ones. for the PCT the smaller relations are modeled after PCTA's section names. in this particular case CA section I extends from Tuolumne Meadows to Sonora Pass. see https://www.pcta.org/discover-the-trail/maps/overview-maps/ for more info.

the PCT sections are members of a superroute relation (a relation of relations) - this way we can still piece together the entire PCT - here it is https://www.openstreetmap.org/relation/1225378