What does it mean when MRI says sequestered disc measuring 7mm by 15mm?

Your disc has ruptured (beyond a herniation, which is full displacement) and the pulpous material (like a jelly doughnut) has become free floating within the disc space. Options are spinal decompression if you have nerve pain or surgery as a last resort to scoop away remaining disc material or allow the nerve root to exit the narrowed cavity without aggravation.