Both the sidechain and backbone h-bond settings default to a value of 1.0. They can be adjusted between 0.0 and 3.0. A value less than 1.0 reduces the normal importance of hydrogen bond, while a value above 1.0 increases it.
These settings are logically similar to Clashing Importance, but clashing importance can only be decreased, where hydrogen bond importance can be increased or decreased.
The "Backbone H-Bonds" setting affects only backbone-backbone bonds, as Foldit scientist rmoretti explained in the "#bugs and feedback" channel on the Foldit Discord server on 29 October 2020:
I wasn't involved with the slider code at all, but taking a quick glance at it, the Sidechain slider will change both the sidechain-sidechain hbond weights as well as the sidechain backbone hbond weights. The Backbone slider adjusts only the backbone-backbone weights (both the alpha-helical short range and the beta+loop long-range ones).
Ligands act like a sidechain. Hydrogen bonds to a ligand will be treated like hbonds to sidechains, and the sliders will affect them. (They use the same score terms.)
So the "Backbone H-Bonds" setting should not affect hydrogen bonding to a ligand.