Get checked. Blood always needs an explanation, but is also common to many benign problems, like hemorrhoids, anal fissures and overall irritated lower GI tract for many possible reasons. However, it is also probably the most reliable sign of serious problems, cancer being one of them, so go and get checked!
Blood in stool. Possibilities can be relatively simple - hemorrhoid or anal fissure - to more complex or serious - ulcer, vascular malformation, cancer. You'd need to be properly evaluated locally to uncover the course.