What “Seen” means
Seen means "I understand", "got it" or "agreed". It acknowledges what someone said or confirms a plan. Said with a questioning tone, it asks "you understand?".
Examples
"Wi link up a six." "Seen."
"We'll meet at six." "Got it."
Mi soon come, seen?
I'll be right back, okay?
How to use it
Common in casual conversation, especially among younger people and in Rasta and dancehall-influenced speech. It's informal, so you wouldn't use it in a job interview.