When the Projection switch is on, then a new mesh created from a topology will have detail from an underlying mesh projected onto it. This is intended to allow transfer of sculpting from an original mesh to a retopologized version of that mesh. This is affected by other controls in the Rigging and Topology submenus, so reading the descriptions for the controls in those submenus is a good idea before using projection.
The primary purpose of Projection Strength is to adjust flipped normals. If you find your vertices are being projected extremely inaccurately when a new mesh is created with projection on, you may have a flipped normals problem, in which case setting Projection Strength to -1 before projecting will fix it. In addition, you can use this setting to create projected details that are more or less amplified, as compared to the original details.
When mesh details are projected, only details within a certain distance of the new mesh’s surface are projected onto it. You can adjust this range upwards to project detail onto parts of your new mesh that have ended up farther away from the surface of the original model. If set too high, you may get details from other areas of the original model projected onto the new mesh.