The Steam Mobile Authenticator is a convenient way to confirm sign-ins to your account. Instead of entering your login and password, you just point your phone camera at a QR code and you’re signed in automatically. The Steam Mobile app also helps you confirm purchases and trades.
To download Steam Mobile, find it in your phone’s store: the App Store or Google Play. Official download links are on the page https://store.steampowered.com/mobile
Note that the developer of the official app is Valve. Don’t download apps from other publishers — they may be phishing apps.