Steam's mobile apps desperately need a redesign, but they're competent enough if you just want to buy a few games and see what your friends are up to. To keep your account secure, Valve currently offers two-factor authentication on desktop via "Steam Guard," which sends a unique code via email. Now, it wants to offer players the option of receiving that code through the Steam app instead.
The feature is called "Steam Guard Mobile Authenticator" and it's live now for a selection of Android beta testers. If you want in, you'll need to join this Steam group and hope Valve takes a fancy to your username. Once selected, you should see the new "Get Steam Guard codes from my phone" option inside the app. Otherwise, you'll just have to hang tight -- Valve can be a tad slow to update its mobile apps, but eventually this security feature should be available to everyone.