A guide in Foldit is a translucent gray shape that can be used to help fold a protein. The shape of the guide can't be changed.
If the guide was created by another player, the current solution automatically switches to from soloist to evolver mode. This intended to prevent players from improving their solo solutions using other players' work.
When the "show stubs" sidechain view is selected, hovering over a sidechain stub on the protein extends the sidechain on both the protein and the guide, allowing players to identify the corresponding segments easily. bands can be created between the protein and the guide to help reshape the protein.
Some puzzles start with a guide. These "quest to the native" or QTTN puzzles include a guide which represents the native shape of the protein. The goal is to match the protein to the guide. QTTN puzzles have largely been replaced by electron density puzzles, but there's still a Quest to the Native puzzle in the intro puzzles.