The bent normal is the direction where most ambient light comes from. The median ray that did not intersect the mesh.
It is not the same as object space normals. A bent normal map could be baked in object space or in tangent space.
Bent normal maps are used to for more realistic realtime shading (GI approximation, specular occlusion)
Some links of where I got the info:
So how would I bake a bent normal map using Blender (with Cycles)?