The backbone is the main form of a protein.
(Sidechains may or may not be shown, depending on the "View Sidechains" option selected in the View Options menu.)
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.