What does this mean? 15mm dominant follicle in right overy?

Ovulation: typically during the first half of the menstrual cycle several eggs start maturing, surround themselves with fluid, become bigger and bigger, and are then called follicles. eventually, one of the follicles takes the lead and is called the dominant follicle. this is the follicle that will release the egg that can then be fertilized. this typically happens when the follicle is greater than 20 mm.