A lesion is simply an abnormality. Depending where it is it can be perfectly safe to leave with no effects whatsoever, or can cause problems such as loss of hearing or loss or balance or headaches or loss of taste or smell....
Some people are born with them and lead perfectly normal lives - the lesions discovered during post mortem and not contributing to the cause of death in any way.
They can be caused by many things including injury; stroke; cancer; mutliple sclerosis.
So as already stated it is impossible to give you a prognosis or the cause.