A blood clot is a collection of blood that has changed state from liquid to gel-like or semisolid. Clotting is a vital procedure that can save you from losing too much blood in some situations, such as when you've been hurt or cut.
When a clot develops inside a vein, it does not usually disintegrate on its own. This may be an extremely hazardous, even deadly, scenario.
A stationary blood clot will not hurt you in most cases, but it may move and become harmful. If a blood clot escapes and travels through your veins to your heart and lungs, it might become lodged and block blood flow. This is a medical situation that requires immediate attention.