Entirely
Completely, Not At All
adverb
If you know all of something's nature then you know it entirely and completely. If you don't, then you know it not at all.
This word is weird. It can mean two opposite things depending on what kind of word comes after it. If it's a negative word, it'll mean "not at all." If it's not negative, it'll mean "entirely." Check out the patterns of use and context sentences for examples.
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.
全然分からない
to not understand at all
全然ない
to not have any of something
全然ちがう
completely different
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.