Every girl has a fixed number of eggs when she is born. These eggs are dormant till she attains puberty. At puberty every month few of the follicles ( female eggs) start developing, one of them matures and releases, whereas the other few degenerate. This way few follicles get used up every month. This goes on till most of the follicles are used up and that is when the woman attains menopause.
AMH is a way to know how much reserve ( potential) does the ovary have for egg production. If it is low it means that the process of childbearing has to be hastened. In that case the couple may need medical assistance. Although women with low AMH may also conceive spontaneously.