What does less than 50% stenosis in bilateral corotid arteries actually mean?

Carotid disease. Any blood vessel may have a "blockage" due to internal clots/plaque or external pressure. The blockage can be graded and thus roughly estimated to be 25, 50, 50-75, more than 75%. Occasionally 60% is used as a cut off. Those numbers help to compare but also help to determine if blockage is significant enough to cause harm. Less than 50% says it is not significant. Test can monitor changes.
Carotid stenosis. Any carotid stenosis means that you have significcant hardening of the arteries that should be treated aggressively. Less than 50% means that the risk of stroke over 5 years is low enough to warrant only aggressive medical therapy. Medical therapy includes blood thinners, cholesterol reducing medicine, and blood pressure medicine. Safe exercise is also very important.