Sidechains determine what other chemicals a protein interacts with and how.
Sidechains also interact with each other, fundamentally determining the three-dimensional structure of the proteins they constitute.
One very important aspect of sidechain function is hydrophobicity, which is how a sidechain interacts with water molecules.
In Foldit, using default view options, sidechains have the appearance of geometric shapes made of short cylinders, which represent the chemical structure of their non-hydrogen elements. Sidechains may be very short, like the single carbon sidechain of alanine, or even invisible, as in glycine. At the other end of the spectrum, sidechains can be comparatively large and complex, like the hexagon plus pentagon sidechain of tryptophan. Sidechains can be manipulated directly by pulling with the mouse, or by using the shake sidechains tool. Sidechains may also adjust themselves during various actions.
Various Foldit view options control how sidechains appear in the game window. Some view options reveal the hydrogen atoms attached to the sidechains.