A method used to separate proteins (in the context of cell biology at least) or other small things based on size. Basically the proteins are flushed through a long tube filled with polymer gel beads, and the order in which they come out the bottom of the tube is proportional to their size.
Although counterintuitive, large proteins actually come out in earlier fractions, and small proteins come out later. This is because the small proteins get caught in and slowed down by holes in the porous beads, while larger proteins cannot fit in the small holes and flow around the beads.