What is a compressed disc?

Back pain? I'm assuming you are referring to spinal condition where you have back pain? Sometimes the spinal columns making up the enitre back (spine) can compress down because of weakness or overweight and cause pains. Xray, MRI or bone scan can show this.
Degenerative discs. A compressed disc is one which has lost some of its water resulting in the disc becoming thinner (less height). It is referred to by doctors as a degerative disc but that doesn't mean that you are aging. Deg. Discs can occur at any age over about 25-30. This is the first step in a process called the degenerative cascade. You will have to look this up due to limited space here.