Histidine (HIS or H) is a weakly hydrophilic amino acid. Histidine's pentagonal shape is unique in Foldit. In contrast, phenylalanine has a hexagonal shape, and is hydrophobic.
In the Foldit Lua interface, the functions structure.GetAminoAcid and structure.SetAminoAcid use lowercase "h" for histidine.
For more, see Histidine on wikipedia.