An ovarian follicle, also known as an oocyte or immature egg, is a fluid-filled sac found in the ovaries that are a part of the female reproductive system. Every month a mature egg is discharged from a follicle during the process of ovulation. Each cycle, many follicles start to mature, but only one will typically ovulate as an egg (rarely, more than one egg is released, increasing the potential for twins). The follicle develops into a corpus luteum following ovulation.
https://pregajunction.com/blog/how-ovarian-and-antral-follicles-relate-to-fertility/