This literally means “blood in a joint.” It occurs most commonly due to trauma. It’s also very common among persons with Hemophilia, a genetic tendency to bleed, but can also be due to several rare diseases. People taking Anticoagulants (“blood thinners”) may be at risk for this, especially in cases of trauma, but mainly only if their level of anticoagulation is abnormally high.
Diagnosis is easy — aspirating joint fluid reveals a lot of blood. Treatment mainly involves rest & comfort measures, until the blood goes away on its own.
See topic Joint Pain — One Joint.