Question

what exactly does it mean to be superimposable?

Answer 1

In Stereochemistry, two molecules are 'superimposable' means that if you place one above the other, they will exactly coincide at all points, and merge to seem as one. Imagine a deck of cards. Take one card, and place another above it. The other card completely covers it, thus the two cards are said to be superimposable on one another.
Try placing one of your hands above the other. They never cover one another exactly. The thumbs point in opposite directions and fingers of different lengths are placed above the other. This is because, our hands are mirror images of each other. Such non-superimposable mirror images are known as enantiomers.