These are prominent veins in the rectum that can bleed. They can be internal or external. They can cause rectal bleeding when passing a bowel motion. Usually the blood is bright red, and can be seen on the toilet paper or in the pan.
Haemorrhoids can be repaired by ligating with a type of rubber band (rubber band ligation), or by surgically removing them (haemorrhoidectomy). Most of the time a colonoscopy will be required beforehand to rule out other causes of rectal bleeding.