A two-dimensional creature living in a 3-dimensional world could not possibly grasp comprehend understand its surroundings. Right?

Correct. 

Picture a flat piece of paper.  A 2D character on that paper  would only be able to look left, right, up, and down (north, south, east, west).  It cannot see anything above or below itself.  Now imagine a 3D sphere hanging over that paper.  The 2D character on the paper cannot see the sphere, let alone comprehend it.  To it, the sphere is invisible.  The 2D character may theorize that such a thing might/could exist (in another dimension), but it cannot prove or disprove its existence.  Now, if the sphere lowers itself into the paper, the 2D character can now see it (at least part of it).  To that 2D character the sphere would look like a thin line, as the rest of the sphere would be below and above the paper - invisible to the 2D character.