This is the 1830th most frequent Japanese word.
急
"急" means "urgent," "swift," or "sudden."
In this sentence, '急に' is used as an adverb meaning 'suddenly' or 'abruptly' to describe an unexpected action.
彼は突然急に立ち上がりました。
He suddenly stood up.
Here, '急いで' is derived from '急ぐ' (to hurry) and used as an action to indicate hastening or moving quickly.
駅まで急いで行きました。
I hurried to the station.
In this context, '急な' is used as an adjective meaning 'steep' or 'sharp', describing the slope.