The picture is not very clear. According to what i can see is it looks like a dental decay to me. You need to visit a dentist and get it evaluated. If it is deep you need to get and X-ray and get it evaluated. If it is not very deep you should get the decay removed and get a composite filling done. If it is very deep and has reached the nerves of your teeth then you should get a root canal done of the tooth. In order to avoid a root canal please do not delay visiting a dentist and getting it treated ASAP.
Hope it was helpful to you.
Benchmark Dental Clinic.