You'll never know until you pry it up. My guess is accesss to some kind of drain.

Whatever it is you need to see what is under there and to me looks like it got wet in a couple of places so you will need to address that before doing anything.

Faced the same issue. For a couple of years, I figured it covered a drain, so I moved water to it in severe storms.

When I lifted the board, it turned out to be a deliberately dug hole in the concrete floor. And, fact, it was the source of the floodwater when the gravel under the floor was overwhelmed.

With the weekend coming up I'll tackle it in case it becomes a bigger than expected project. The water on the floor was from me washing it, I should have mentioned that. I suspect Worthy is on to something, I'm smelling a very damp musty smell coming from it. Thanks everyone :)