If you want to enable the setting, it's under

Settings > Profile > Advanced Settings > Mark this account as a cat

(wording/location may vary if you're using a misskey fork but it should be under the profile settings somewhere)

This will give your icon cat ears in public timelines and any text you write will be converted to catspeak. Works better in Japanese, which is the language it was designed for, but in English words like "now" get converted to "meow" for example.