Question

What is the significance of the dashed bonds in this ball-and-stick model?

Answer 1

On paper it is hard to represent three-dimensional molecules. The dashed bond projects into the page. The wedge projects out of the page.

Explanation:

Of course we cannot depict a three-dimensional tetrahedron on two dimensional paper. When we draw a stereocentre `CR_1R_2R_3R_4`, two of the `C-R` bonds are presumed to lie in the plane of the page, and in this scenario one of the `C-R` bonds projects out of the page, and the remaining `C-R` bond projects into the page.

You will always be permitted to bring molecular model sets into examinations (in fact most organic professors would have on their desks a set with which they would routinely fiddle), of course you have to learn to be able to represent these three dimensional structures on two dimensional paper. The dash and wedge notation is such an attempt.