"Altcoin" is a loose catch-all term for any cryptocurrency that isn't Bitcoin — from large, established networks like Ethereum, to thousands of smaller, less proven projects. The term says nothing about quality or legitimacy on its own; it's purely a category based on not being Bitcoin.
Because the altcoin category spans everything from major infrastructure projects to speculative tokens with little real usage, treating "altcoin" as a single investment class is a common beginner mistake. Each project needs to be evaluated on its own merits.