"Seeing" someone means dating them or in a romantic relationship with them. For example:

Are they seeing each other?

I was interested in her at the time, but she was already seeing someone.

You might not use "seeing ___" when talking about the past:

That was back when they were dating.

You also wouldn't use "seeing ___" to talk about a couple who has been together for a long time:

They've been together for, what, like three years now?