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.
Histidine's sidechain contains two nitrogen atoms, one of which which can act as a hydrogen bond acceptor. The other nitrogen atom can act as a hydrogen bond donor. See the Sidechain Bonding Gallery and the Sidechain Bonding Table for other sidechain donors and acceptors.
For more, see Histidine on wikipedia.