Blood contains many small, granule-like objects called red blood cells. These red blood cells have a lifespan of only about 120 days, so every day the old ones are destroyed and new ones are made.
The dead red blood cells are not stored in the body in their original state, but are discarded outside the body, mixed in with feces.
The red blood cells are transformed in various ways in the body before they are thrown out with the feces. One of these changes in the shape of red blood cells is the urobilin mentioned earlier. This urobilin is brown in color, which is why the feces are brown.