Entirely, Completely, Not At All
adverb
It's all nature out there. It's entirely nature and not at all city. This word is weird. It means two opposite things. Thanks Japanese! Just look out for a negative word after the 全然. If it is negative it's "not at all" if it's not negative it might be "entirely."
ぜんぜん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
え、全然そんなことないよ。
Eh, it's not like that at all.
「すみません。今、ちょっといいですか？」「はい、全然いいですよ！」
"Excuse me. Can I ask you something right now?" "Yes, of course, go ahead!"
彼が何を言っているのか、全然分かりませんでした。
I had no idea what he was talking about.