A Steam Web API key is a long code tied to your Steam account. Developers register one to build tools that talk to Steam, such as inventory viewers or trading bots. Registering a key takes seconds on the Steam Web API key page and Steam does not send a notification when one is created.
For a normal CS2 player the key has almost no use. You do not need one to trade, to sell on a marketplace or to use a float checker. The only reason to care about it is security: a key on your account gives its holder the power to see and cancel your trade offers.
What can someone do with my Steam API key?
With your key, a script can watch your account for outgoing trade offers, cancel them the instant they appear, and read the item list so it can copy the offer. That is the engine behind the API key scam. The key does not let anyone log in, and it cannot create trade offers on its own, but combined with a lookalike account it is enough to steal skins.
How do I check and revoke my Steam API key?
Go to steamcommunity.com/dev/apikey while logged in. If you never registered a key, the page shows a form to create one and nothing else. If a key is listed and you did not make it, click Revoke My Steam Web API Key right away. Then change your password and deauthorize all other devices, because the scammer got the key by logging in as you.
Check this page after any unexpected login, after clicking a link you regret, and before selling anything expensive. The API key scam guide shows the page and the warning signs.
If you genuinely use a tool that needs a key, keep the key private, and revoke it when you stop using the tool. A domain name is required when you register, and scammers often use random text there, so a strange domain on your key page is another red flag.