Learn more about Steam Guard Mobile Authentictor:
How to enable Steam mobile authenticator? We take ios for example.
1.First, go to App Store，download "Steam Mobile" app.
2.Log in with your steam account，open the menu in the left，and click "Steam Guard"
4.We used to check our trade by emails. Now, we need to "Get Steam Guard codes on my phone".
5.Click "next" to move on.
6.The steam mobile authenticator will be activicated after you enter the SMS code steam send you.
7.Steam will send you a letter about the successful mobile authenticator adding.
8.After you activated Steam mobile authenticator, everytime you want to login through steam, you'll need to enter the code here.
9.You'll need the code in steam app when you login a new site