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. 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 sends codes to your registered email address. Mobile Steam Guard uses the authenticator in the Steam app, which also confirms trades and Market listings. Only the mobile level removes trade holds.
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. If you use email codes only, you can still trade, but every trade and listing gets a 15-day hold. With the Mobile Authenticator active for 7 days or more, trades go through without a hold.
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 steamcommunity.com or store.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. Move the authenticator to a new phone through the app rather than removing it.