Earlier this month, Valve announced it was working on a Steam Link app for Android and iOS. Much like the separate Steam Link box, it would allow you to stream games from a local PC. The beta version was expected to arrive on May 21, but it has now arrived a few days early.
Once you install it, you have to pair it with a PC running Steam on your local network (and possibly install a driver or two). Then you can play PC games from your Android device, assuming the network connection is fast enough. The app supports the Steam Link controller, or whatever other Android-compatible controllers you happen to have lying around.
You can grab it from the Play Store below, or from APKMirror. In addition to phones and tablets, it also shows up as compatible with Android TV devices and Chromebooks.