I bought a model that has .obj, .fbx and .max formats. I use Mac, so I'm not able to use .max, so I decided to import .obj in Blender. After I imported the file, I saw that a part of it hidden as if there was a gray wall or a portal to another universe. It's hard to explain it, better check the gif:
I'm totally new to Blender, so I have no idea what it is.