DNA, when cut by , vary in length. In order to separate them, you would have to run a sample of DNA through a gel using electrophoresis.
A saline buffer is poured into the gel box (which has the gel) and DNA is loaded into the gel (using micropipettes). Electricity is run through the gel (hence the need for the saline buffer) and the negatively charged DNA is pulled towards the positive cathode of the gel box. The gel is porous, which allows DNA to move its way through the gel. Longer chunks of DNA move through the gel much more slowly as it is larger and gets “stuck” in the gel. Smaller chunks can more easily go through the gel as they can fit through the pores of the gel more easily.
In the picture, the larger pieces are seen closer to the top and are a bit brighter, and the smaller pieces are seen closer to the bottom and are a bit more faint.