Early Access is just "release". Only the devs openly admit ahead of time the game is buggy and unfinished, and promise - as always - to fix it up and add the missing parts.

Often they do. Sometimes they don't.

TBH it's ultimately nothing but a shitty buggy release, but the honesty of making that known ahead of time buys a whole lot of goodwill. It should be the default, that any publisher releasing a game that is not finished - so most AAA nowadays - marks it as Early Access, openly declaring the unfinished part.

It's also very different from a beta version, which is usually feature and content complete (otherwise it's generally called an alpha). Early Access versions are often very early in the development process, they're feature-complete-ish, but never or rarely content complete, usually just starting out on that. This works exceedingly well for games that need "just more stuff", but can miss the mark on games that need underlying systems reworked as this ires the existing playerbase and splits it.