Bits are a staple of Twitch streaming. They're a way to back a streamer if you can't afford a full subscription, or want to offer some additional support. Until now, though, you couldn't easily buy them on your phone -- you had to either go through the web or wait until you got to a PC, by which point it might be too late. That won't be a problem for much longer, as Twitch is rolling out Bit purchases in its mobile apps. If you want to celebrate a streamer's victory, it's now just a few taps away... for an extra fee.
Source: Twitch Blog