Steam Guard is the extra login check on a Steam account. When it is on, a password alone is not enough to sign in from an unknown device. Steam also asks for a short code that goes to your email or to the Steam Mobile app, as the Steam Guard support page explains. The mobile version is called the Steam Guard Mobile Authenticator and it is the one that matters for trading.
Steam Guard has two levels. Email Steam Guard is on by default once your email is verified and sends codes to that address. Mobile Steam Guard uses the authenticator in the Steam app, which Steam calls the best level of account security. The Mobile Authenticator also confirms sign-ins, trade offers and Market listings from your phone.
What is the difference between Steam Guard and the Mobile Authenticator?
Steam Guard is the general name for the login check. The Mobile Authenticator is the phone-based version of it. Steam's rule for games that use trade holds is that trades and Market listings made before the authenticator is added, or during its first 7 days, are held for 15 days. Counter-Strike 2 items are the exception: Steam says CS2 items are not subject to trade holds because Trade Protection covers them instead. Email-only accounts still face other limits, such as no trading or Market use from a new device for 7 days, and most marketplaces require the Mobile Authenticator before they will trade with you.
Does Steam Guard stop scams?
It stops the simplest one: a stolen password. It does not stop you from typing your password and code into a fake Steam login page, and it does not stop a scam that uses your own Web API key to redirect trade offers. Steam Guard also cannot help if you approve a trade on your phone without reading it.
So treat it as one layer. Keep Steam Guard on the mobile app, check that the login page address is one of the official Steam domains (steampowered.com, store.steampowered.com, steamcommunity.com or help.steampowered.com) before typing anything, and check your API key page after any suspicious login. The account security guide walks through all of these steps.
If you remove the Mobile Authenticator, Steam blocks trading and Market use on that account for 15 days, and that applies to CS2 players too because it is an account restriction, not an item hold. Move the authenticator to a new phone through the app instead, which brings only a 2-day restriction.