What does the sieving effect in gel electrophoresis refer to ?

The DNA fragments separate (resolve) according to their size through sieving effect provided by the agarose gel. Hence, the smaller the fragment size, the farther it moves.

In gel electrophoresis, DNA fragments are separated based on their size by passing them through an agarose gel matrix. The agarose gel acts like a sieve, where the DNA fragments move through the pores in the gel when an electric current is applied. Smaller DNA fragments encounter less resistance as they pass through the gel, allowing them to travel further than larger fragments. 

This movement occurs because smaller molecules can navigate through the tiny pores of the gel more easily, while larger fragments are hindered and move more slowly. As a result, the DNA fragments get separated into bands, with smaller fragments located farther down the gel and larger ones remaining closer to the starting point. This process is essential for analyzing DNA by size, such as in DNA fingerprinting or PCR product analysis.