This is how it looks like on erection.
There are two different types of things this can be, one is a penile cyst, the other type is a blocked skin pore, infected Sebaceous gland or hair follicle that is either blocked with an ingrown hair or has otherwise become infected.
It is best to try to resolve the second type of causes, to do this you need liquid antiseptic (not cream or ointment versions) and apply some undiluted to the bump 3 or 4 times a day. It should begin to reduce in size after 3 or 4 days - but keep applying the liquid antiseptic 1 or 2 times a day until it is completely healed.