How is it possible to sort different lengths of DNA strands using gel electrophoresis?

During gel electrophoresis, a DNA strand is cut into fractions during a step of gel electrophoresis called fragmentation where a strand of DNA's nucleic acids are separated by applying a gel substance with an electric field. From there the nucleic acids are moved through agarose and other substances. Now that the DNA strand has been fragmented it's length can be estimated.