The backbone is the main form of a protein.
In a protein, the backbone is a chain of amino acids linked together end-to-end via peptide bonds, forming a polypeptide (a type of polymer).
DNA and RNA, which appear in some Foldit puzzles, also have a backbone, although the chemical details are different. DNA and RNA are chains of nucleotides.
Except for glycine, amino acids have a sidechain, which projects from the backbone. The sidechain is what makes each amino acid unique. The backbone atoms are the same for each amino acid.
(Sidechains may or may not be shown, depending on the "View Sidechains" option selected in the View Options menu.)
There are many ways to manipulate the shape of the backbone in Foldit, including Nudge, Rebuild, Tweak, and Wiggle.
Foldit does not allow the peptide bonds of the backbone to be broken. A cutpoint can be used to temporarily split the backbone, but all cutpoints must be closed for the protein to score.
Design puzzles usually allow players to insert or delete segments, in effect adding or removing peptide bonds.
Some Foldit puzzles have two or more separate sections. It's not possible to join these sections to form a single backbone.
See Protein backbone, RNA backbone, and DNA backbone for an atom-by-atom view of the different types of backbone.