A DNA ladder is a solution composed of DNA molecules of varying lengths. It is widely used in gel electrophoresis as a reference point or molecular-weight size marker to determine the approximate size of unknown DNA fragments. 

In gel electrophoresis, DNA fragments are separated based on their mobility through a gel that has an electrical charge running through it. Running a DNA ladder alongside an unknown PCR product in an agarose gel, allows researchers to estimate the size of the unknown DNA fragment by comparing it to the nearest-size band in the ladder lane.