BTTV (otherwise known as BetterTTV) is a third-party browser extension that allows people to use more emotes in chat, than what officially Twitch gives a channel. There are both Global BTTV emotes and Channel specific emotes (each streamer can upload personal BTTV emotes for their own channel). There are both Global BTTV emotes and Channel specific emotes (each streamer can upload personal BTTV emotes for their own channel).
An easy way to check if BTTV emotes are enabled for you, is if you see "monkaS" as a text in chat and not the emote.
Visit BTTV's website and download the browser extension for your specific browser and then.
Once BTTV is installed in your browser, there are a lot of settings you can.
However, to enable emotes, you need to click on the ⚙ icon and then on "BetterTTVSettings".
In settings enable both "BetterTTV Emote" and "BetterTTV GIF Emotes".
Next you can either write "monkaS" or use the new BTTV emotes menu to use the new emotes.
Now you should be able to see BTTV emotes in chat as well.