The full form of noob is Newbie. It’s used on Internet.

an inexperienced newcomer to a particular activity.

Newbie, newb, noob, or nub is a slang term for a novice or newcomer, or somebody inexperienced in a profession or activity. Contemporary use can particularly refer to a beginner or new user of computers, often concerning Internet activity, such as online gaming or Linux use. Depending on the context and spelling variant used, the term can have derogatory connotations—but is also often used for descriptive purposes only, without any value judgment.