DNA fragments make the lines on an electrophoresis gel.
Fragments of DNA from each sample are placed each placed in a different well/slot.
A current is then run through the gel. The DNA migrates through the gel towards the positively charged side because DNA fragments are negatively charged.
The smaller fragments travel farther because they travel through the gel more easily (as seen in the picture from Encyclopedia Britannica).
Fragments of the same size group together at different areas of the gel making those lines.