Tautomers are different forms of a molecule which have the same chemical formula and overall structure. Tautomers are a type of isomer.
In Foldit, the amino acid histidine has two tautomers. In one tautomer, the nitrogen at atom 10 acts as a hydrogen bond donor, and the nitrogen at atom 7 acts as a hydrogen bond acceptor. In the other tautomer, the atom 10 is an acceptor, and atom 7 is a donor.
In histidine, the difference between the tautomers is a where a hydrogen atom, often referred to as "proton", is located. The nitrogen bonded to the hydrogen becomes the hydrogen bond donor.