What does it mean to have a sub-acute annular tear at s1 vertebrate?

Disk injury: Technically is not not a tear of the vertebra (a bone), but the disk between two bones. The outer disk (annulus fibrosis) is made of layers of collagen (a material). Think of them as threads woven in layers. They allow for some flexibility and movement between the bones. With aging and time some of the fibers can tear. If nerve fibers grow into the area, it might be a source of pain.

Annular tears are : Cracks in the outer layer of the disc that can occur with age and degeneration or with an injury which can lead to a part of the disc herniations out through the tear. Most cause no symptoms but some can lead to back and/or leg pain. Subacute is a term referencing the degree of signal intensity of the tear on a MRI possibly suggesting the age of the tear.