It's a polite and formal way to compliment someone on their good looks.
Here's a breakdown of the phrase:
* 미남 (mi-nam): handsome man
* 이시네요 (i-si-ne-yo): a polite and formal way of saying "you are"
It's important to note that the phrase "미남이시네요" is used exclusively for men. For women, you would use "미녀이시네요" (mi-nyeo-i-si-ne-yo), which means "You are beautiful."