It is where the blood vessels are fragile and malformed, they are more likely to swell and burst/rupture. This can cause stroke, or haemorrhage (bleeding inside your head). Varicose veins can be seen in legs, they can swell and cause problems, so can other blood vessels. The problem is that inside the brain or heart they can bleed or form a clot as things are not flowing smoothly. How much of a problem this is depends on where this is, most people don't get any symptoms until they are over 40 or 50, even then it is not a common condition