She is using gel electrophoresis, where a DNA sample with DNA molecules of varying length are ran through an agarose gel to separate them into bands by size from longest at the top to shortest at the bottom.
Gel electrophoresis is a technique that is used to separate DNA fragments based on their size.
In this technique, DNA samples are loaded into the wells at one end of a gel, and then an electric current is applied. DNA fragments being negatively charged move towards the positive electrode.
Since, all DNA fragments have the same amount of charge per mass, small fragments move through the gel at a faster rate than large ones.
