Salesforce Trailhead is a place where the Salesforce community usually starts to learn about itself.
Judging by Wikipedia, the definition of a Trailhead goes something like this: “A trailhead is a point at which a trail begins, where the trail is often intended for hiking, biking, horseback riding, or off-road vehicles.”
We could agree on how Salesforce Trailhead is the point at which the Salesforce trail begins.
Or as Salesforce likes to put it:
‘Trailhead is a fun way to learn Salesforce.’