FSH = Follicle Stimulating Hormone. It is made in your pituitary gland, and tells the ovaries to grow follicles. When your ovaries function well, they don't need much FSH to start growing follicles. At some point, the ovaries are getting older and then they need more FSH to grow follicles, so more FSH is being made.
This means that a low FSH level means well functioning ovaries, while a high FSH level means that the ovaries aren't functioning as well anymore. The number often quoted is 10. When the number is smaller than 10, your ovaries are still in good shape. 10 or higher means that they need more stimulating to get the follicles growing, so aren't functioning as efficiently anymore.
Hope that helps!