Authentication
Every request carries your key in the Authorization header.
Authorization: Bearer rv_…A key belongs to one account and can do what that account can on the site: read its data, list and buy items, spend its balance. Keep it on your server, and out of repositories and client apps.
Getting a key
Make it on the API key page. An account has one key. Rockvel shows it once and keeps only a hash of it, so a lost key can't be restored, only replaced. A new key ends the old one at once, and so does deleting it.
What a key can't do
Sign in, or make, replace and delete keys. These take a browser signed in through Steam, so a stolen key can't lock you out: delete it on the site and it stops working at once.
When a key is refused
A missing or unknown key gets 401 with WWW-Authenticate: Bearer. A session cookie doesn't count here: the site and the API are separate ways in.