An adverb meaning that something is already completely finished or a state has fully progressed; a strengthened form of 'already'.
もうすっかり春らしくなったね。
It's already completely like spring, isn't it?
彼はもうすっかり回復して、普通の生活に戻っている。
He has already fully recovered and returned to his normal life.
引っ越してからもうすっかり新しい街に慣れた。
Since moving, I've already completely gotten used to the new town.