A watertight mesh is a closed surface with no holes and no self-intersecting or non-manifold edges: if you filled it with water, none would leak out. Slicers need this because they must decide what is inside the solid to fill and what is outside. Raw AI-generated meshes are often porous shells, which is why they get rejected. 3D Mate exports watertight STL and 3MF so the file slices without repair.
A slicer converts a surface into solid layers of plastic or resin. To do that it needs an unambiguous inside and outside, which requires a closed, manifold mesh. Holes, boundary edges, or edges shared by more than two faces (non-manifold) make that ambiguous, and the slicer warns or refuses.
Image-to-3D models reconstruct a surface that can have small holes, thin shells, or internal walls. It looks fine on screen but is not a solid. That is the gap between a pretty render and a printable object, explained in the figurine guide.
When you export STL or 3MF for printing, 3D Mate closes holes and makes the mesh manifold, so it comes out watertight, upright, and at real millimetres, ready to slice in PrusaSlicer, Cura, or Bambu Studio with no repair tool. See the 3D printing page.