MRI ARTHROGRAM LEFT HIP ; A very small subchondral cyst was noted along the lateral roof of the right acetabulum What does this mean?

Early DJD. Subchondral cysts mean cysts in the bone underneath the cartilage, and are a reaction of the bone to degenerative and stress related changes about the joint space. These are usually seen with some cartilage damage and narrowing of the joint space adjacent to the cyst. Many people are asymptomatic with small cysts.
Not serious. an MRI is a very good test for bone and connective tissue. if it is read as a cyst by a radiologist experienced in MRI, it is not anything to worry about.