Ovarian vein reflux means blood flowing the wrong way — down the ovarian (gonadal) vein into the pelvis, because the one-way valves that should prevent it have failed. The pooled blood stretches the pelvic veins into varices, and the result is the dragging, standing-worsened heaviness of pelvic congestion.
The left side is affected far more often, for a plumbing reason: the left ovarian vein drains into the renal vein at a right angle (the right one joins the main body vein more favourably), and left-sided compressions add pressure. Pregnancy is the classic trigger — ovarian veins stretch massively to carry pregnancy blood flow and don't always recover their valves.
On reports you may see it graded by vein diameter (dilated is commonly taken as around 6–8 mm or more) plus the direction of flow on duplex ultrasound or a venogram. Treatment that targets it directly is embolisation.