Foldit classifies all amino acids as either hydrophobic or hydrophilic. Hydrophobic amino acids are generally found in the core of the protein. When the "Hydro" coloring option is selected, hydrophobic amino acids appear in orange.
The name "hydrophobic" means these amino acids tend to move away from water. The hiding score component for hydrophobics indicate how well they are hidden from water molecules on the outside of the protein.
In Foldit, the hydrophobic amino acids are:
Not all amino acids are equally hydrophobic, there's a hydropathy index that estimates how hydrophobic each acid is in typical conditions. The hydropathy index can be found in the table on the amino acids page.