Oocytes are the female gametes, cells that, upon being fertilised by the sperm, will produce an embryo. In humans, the number of oocytes that females have throughout their lives is not unlimited; they are born instead with a limited number. These oocytes gradually run out in each menstruation until their supply becomes completely exhausted at menopause.
It is said that women suffer from premature ovarian failure when menstruation ceases because the oocyte supply has become exhausted at an earlier age than the age when the natural decline of ovarian function occurs (at 50, approximately). In cases where oocytes are still produced in women suffering from premature ovarian failure, they do not respond satisfactorily to the hormones responsible for their development and maturation.
Premature ovarian failure may have important consequences for women:

Obviously, in their fertility: These women cannot conceive naturally and require assisted reproduction techniques in order to bear offspring. These patients also have a high risk that their oocytes will not respond to the In Vitro Fertilisation cycle, thus putting the success of the treatment in danger.
On the other hand, the absence of female hormones due to low ovarian activity has serious consequences for health, mainly in cases of cardiovascular disease and osteoporosis.