Immediate, Instant
adverb
If you see 即 before a word, you know that it's probably something that happens really fast or instantaneously. That's why this is immediate or instant.
そく
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
すごく可愛いテディーベアを見つけたので、即買いしちゃいました。
I found a super cute teddy bear and immediately bought it.