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.
Histidine has two tautomers, which differ only in the placement of one hydrogen atom. There are two nitrogen atoms, numbered atom 7 and atom 10, except at the C terminal, when they become atom 8 and atom 11.
A hydrogen may be attached to either atom 7 or atom 10, but not both. The nitrogen with the hydrogen attached becomes a hydrogen bond donor. The other nitrogen can act as a hydrogen bond acceptor.
In Foldit, actions which change the position, such as using the shake tool, can change histidine from one tautomer to the other. The tools select the best-scoring position, regardless of the tautomer.
Manually dragging a histidine sidechain to different positions can also change the tautomer.
To see the different tautomers, select the "stick + polar H" protein view under View Options. This view will show a white hydrogen atom next to one of the blue nitrogen atoms in a histidine sidechain. Selecting "Show bonds (sidechain)" and "Show bondable atoms" highlights the acceptor nitrogen with a red sphere, and the donor nitrogen with a blue sphere.
See Are all Histidines in Foldit the same? for more on histidine's tautomers.
See also Histidine on wikipedia.