It is hard to give an exact definition to this particle but it has the meaning of “you know it”.
It is commonly used when you talk to a friend or someone whom you know.
When the statement that you’re about to make is a fact that is known by both of you, or at least that’s what you think, you would attach 잖아 behind.
Example: 왜 밥을 안 먹나? 나 직접 만들었잖아.
English translation: Why are you not eating? I personally cook this (and you know it).