"Long time no see" is an informal phrase used to greet someone you haven't seen in a long time. It expresses the idea that it's been a while since you last met the person. Despite its ungrammatical structure, it's a commonly accepted colloquial expression in English.
For example:
Friend A: "Hey! Long time no see!"
Friend B: "Yeah, it’s been ages!"
The phrase is often used in a friendly, casual context.