I feel dumb, but...

You're not dumb at all. It's a good question.

For me, it's whether someone is open to & can see more from life than just the utilitarian. Not that it's dismissing the utilitarian, as that's a necessary part of life—but the film is simply reminding us that the things that finally & truly matter to us as human beings are often immaterial & emotional—love, faith in each other, family, memories, desires & dreams.

As Mr. Keating puts it in Dead Poets Society:

"We read and write poetry because we are members of the human race. And the human race is filled with passion. And medicine, law, business, engineering, these are noble pursuits and necessary to sustain life. But poetry, beauty, romance, love, these are what we stay alive for."

Ray & Annie still remember & know what it is we stay alive for, so they can see the players. Mark has forgotten, so he can't ... until he's shocked into remembering, at which point he can see the players. And like any convert, he's zealous about what he's suddenly (re)discovered, insisting that Ray must keep the field & not sell it, not under any circumstances.

As the alien captain says to Picard in the episode "Darmok": “Sokath, his eyes open!” Or in this case, "Mark, his eyes open!"

I doubt that Mark's partners would ever be able to see the players, though.

I see this differently than Owl does.

The way it seemed to me was that Mark was not any different than anyone else (indeed, he seemed like a decent sort who was only trying to protect his sister from this nut who built a baseball field he couldn't afford), but at the moment that Doc Graham crossed the foul line and suddenly appeared in time to save the choking girl, the ballplayers became visible to everyone. Indeed, this would seem to be necessary if all those people showed up to Ray's field were to see them.