The Steam Guard Mobile Authenticator is the part of the Steam Mobile app that protects trading. Steam's authenticator FAQ explains that it creates a new code every 30 seconds, each code works once, and the app shows a confirmation screen for every trade offer and Community Market listing you make. Your phone only needs the correct time, not an internet connection, to make codes.
Setting it up takes a few minutes. Install the Steam app on your phone, sign in, open Steam Guard and follow the steps. Steam gives you a recovery code at the end. Write it down somewhere safe, because it is the fastest way back into your account if you lose the phone.
How long after adding the Mobile Authenticator can I trade?
For games that use trade holds, Steam holds any trade or listing created before the authenticator is added, or during its first 7 days, for 15 days. After those 7 days, new trades and listings go through with no hold. CS2 is different: Steam says Counter-Strike 2 items are not subject to trade holds because Trade Protection covers them instead, so a CS2 trade moves at once and can be reversed for 7 days. Account-level restrictions still apply to CS2 players: a device that is new to Steam Guard cannot trade or use the Market for 7 days unless the authenticator has been on the account for at least 7 days. The trade hold calculator works out the dates.
What happens if I remove the Mobile Authenticator?
Removing it lowers your account security, so Steam blocks trading and Market use for 15 days. If you are changing phones, use the Move Authenticator option inside the app on the new device instead. Steam says a transfer brings only a 2-day trade and Market restriction, and it recommends transferring rather than removing.
The confirmation screen is your last defense against a bad trade. It lists the items you give and receive. Scammers count on people tapping confirm without looking, especially in API key scams where the items look right but the receiving account is wrong. Read the name of the other party every time.
Keep the app on a phone you control and lock the phone. Anyone with your unlocked phone can approve trades.