Yeah, those pics tell me nothing. But if this gap or irregularity or whatever it is has appeared recently, then something has changed; the stand itself, the floor, whatever. And that change may be ongoing and worsening. I would get that tank emptied and off that stand immediately.
I'm pretty sure that is a particle-board stand. I still cannot believe how many of those things are made and sold for aquarium use. In terms of the completely inappropriate application of the material for the purpose, they make about as much sense to me as building a stand out of stacked cubes of sugar. Might be fine until it gets wet, but then...
I dunno, maybe I'm the only klutz on the planet who always somehow manages to have a splash or spill on my aquarium stands.