When you change the world axes in the main sketch, or in a group, a work plane is associated with the X and Y axes. A work plane is a virtual 2-dimensional surface used as the origin for sketching elements. This work plane will stretch out to infinity and provide a surface to sketch on without snapping to elements behind it.
Changing Work Planes
After you set the world axes a light blue plane appears.
Rotate the world axes to the plane of another object in the sketch. This will align the active work plane to correspond to this face. You can now sketch, place elements, and modify elements relative to the new XY plane, rather than the default ground plane. You can orbit behind or under the grid and work planes to snap and inference to elements behind it
If you do not see the grid lines then you need to turn them on to benefit from work plane based sketching ( > Visual Style > Display Grid)