May-Thurner syndrome is a plumbing problem of position: the left common iliac vein — the main vein draining the left leg and pelvis — runs underneath the right iliac artery, and in some people the artery pinches the vein flat against the spine.
The classic signature is left-sided: one leg heavier or more swollen than the other, left-dominant pelvic symptoms, and — the serious version — a deep vein thrombosis (DVT) in the left leg, sometimes the event that finally gets the compression found. Mild compression is common and often meaningless; what matters is whether it measurably obstructs flow, which is why confirmation uses IVUS and pressure measurements rather than a picture alone.
Treatment, when warranted, is usually a stent holding the vein open — see living with a venous stent — and when May-Thurner coexists with pelvic congestion, treatment order matters.