A rectangle is a four sided object that has all four angles at 90 degrees (aka right angles). Imagine that the cards are designed as such they do not have their corners rounded off, but each corner is a 90 degree angle. Stacking 52 congruent rectangles leads to a 3D solid known as a rectangular prism. The base is a rectangle that is congruent to any of the 52 cards. This stack must be aligned and not allowed to have any of the cards offset (ie forming a sort of "leaning tower"). You can think of a "rectangular prism" as a "box" which is probably a more common term for it; however, "rectangular prism" is probably what your teacher will want since it likely involves geometric terms you've learned in your lessons.
