[Question] I haven't played a video game in over a decade. Now I want to buy one. It's on 20% sale on Humble but you buy it from Steam.. How does that work?

I'm looking for the practicalities here. I've never used steam, but I can understand that you buy a game and download it. You wait for the download to finish and than install the game. Easypeasy. How does that work with Humble? Do I have to enter a key or something?

I want to buy Timberborn. It says it's "early acces" does that mean it's not the full game and I should instead just buy it via steam?