It is a symptom of disease involving the gastrointestinal tract.

Many possible causes include hemorrhoids, peptic ulcer, tears/ inflammation in esophagus, diverticulitis, ulcerative colitis, crohn’s disease, colonic polyps, cancer in colon/stomach/esophagus.
Might present as blood in vomitus/ stools or dark tarry stools. Sometimes the amount of bleeding can be so minute that only a lab test can detect it ( occult bleed).

The test used to look for the cause is Endoscopy
( Upper GI Endoscopy- to look at esophagus, stomach, small intestine.
Colonoscopy- to look at large intestine) .
Treatment involves replacing the blood lost and treating the underlying cause of bleed.