Lumbopelvic refers to the lumbar spine where it attaches to the pelvic bones. Lumbopelvic rhythm is used to describe the coordinated movements of the joints of the pelvis and lumbar spine during trunk flexion.
As you bend forward the spine and pelvis move in a smooth path to make flexion possible. It's actually a bit more complicated than just described because the pelvis also rotates around the hip joints during forward flexion.
One other place in the body where the bones move together in a coordinated fashion is the scapula (shoulder blade) on the rib cage as the arm moves overhead. This is called the scapulohumeral rhythm. For every two degrees of shoulder motion, the scapula moves one degree in a tilting motion. This rhythm allows the arm to move smoothly up overhead.
Restoring the lumbopelvic rhythm often involves working with the joints and muscles of the spine, pelvis, sacrum, and hips. If the muscles in any of these areas are weak or in spasm, they can prevent this natural rhythm from occurring. The same is true if the joints are out of alignment or "stuck" and not moving smoothly.