The Steam Guard Mobile Authenticator is the part of the Steam Mobile app that protects trading. It creates a new login code every 30 seconds and shows a confirmation screen for every trade offer and Community Market listing you make. Without it, or during its first week, Steam holds trades and listings for 15 days.
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?
Any trade or listing created before the authenticator is added, or during the first 7 days after adding it, is held for 15 days. After those 7 days, new trades and listings go through with no hold. The 7 days are counted from the moment the authenticator was enabled, so you can set it up now and trade freely a week later. The trade hold calculator tells you the exact date.
What happens if I remove the Mobile Authenticator?
Removing it lowers your account security, so Steam blocks trading and Market use for 15 days. This also happens if you deauthorize all devices in some cases. If you are changing phones, use the option inside the app to move the authenticator to the new device, which avoids the block.
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.