In online jargon "key" or "major key" is slang to describe anything that's essential or important. For example "Don't forget da party pack dude, dat would be key."

In technical terms, "key" is used to describe a long binary number used to encrypt data. To unlock encrypted content, you must find the precise mathematical combination that makes up the key (for example, two prime numbers that when multiplied produce the key). The longer the key, the more bits it has and therefore the more possible combinations of bits. This makes it more difficult for someone to guess the right combination. Key length alone does not make encryption invincible; an algorithm also makes it difficult for a hacker to discover and exploit any patterns in the encryption.

NetLingo Classification: Technical Terms