Gel filtration chromatography is a laboratory technique to separate biomolecules by size. The apparatus can be thought of as nothing more than a column filled with porous beads. As a solution of molecules travels through the column, the smaller molecules get temporarily stuck in the beads, while the larger ones pass right by them. As more solvent is poured through the column, the smaller molecules will find their way out. This means that the first fraction collected will have the largest molecules, and later fractions will have smaller ones. This is not to be confused with gel electrophoresis where smaller molecules travel faster than larger ones.
See also: Chromatography, gel permeation chromatography