Twitch users on iOS can now subscribe to their favorite streamers directly from the app. It'll offer the same benefits of subscribing on desktop, including ad-free viewing, sub badges, channel emotes and sub-only chat, as well as supporting the partner or affiliate monetarily.
Subscriptions work differently on iOS than other platforms on which Twitch is available. You'll need to buy an iOS Sub Token through the app, which is good for a Tier 1 subscription for a month. You can redeem up to 12 at a time per streamer to keep a subscription to them going for a year.
The tokens cost a little more than desktop subscriptions to account for Apple's App Store fees. They're $5.99 instead of $4.99, but you can buy two for $8.99 until the end of the month. It doesn't seem you can use Twitch Prime subscriptions through the iOS app either.