Bleeding gums can indicate a few things, often it can be due to the gums experiencing trauma during brushing. For example if you brush with a hard bristle toothbrush and brush roughly against your teeth and gums, the bleeding is likely to be from damage done to gums by that process. Try to avoid brushing too hard as it can cause your gums to recede and lead to other issues as well as an unsightly smile!
A buildup of plaque or bacteria on the surface of your teeth can also lead to gum inflammation which can cause bleeding gums, this is called Gingivitis. Inflammation of your gums can cause spontaneous bleeding as well as bleeding when you brush.
Keep in mind that bleeding gums can also be a sign of other health related issues and it is strongly recommended that you visit your dentist if you notice your gums are bleeding!