Ovulation is the rupturing of a follicle with subsequent release of an egg.
Women are born with 1-2 million eggs. These lay dormant in the ovaries wrapped in a layer of cells. A FOLLICLE is the name given to an egg surrounded by the layer of cells (known as granulosa cells).
Each month, out of the entire supply of follicles, a small group of them (known as the recruited cohort) begin to grow. Most of the time, under natural conditions, only one of the follicles grows to the mature state. This is called the dominant follicle. The rest of the follicles that month only grow partially and are wasted, lost forever. Under conditions when a woman takes fertility drugs, many more of the original group of follicles can be rescued so that they, too, have a chance to initiate a pregnancy.
From many lectures to my medical students, one analogy that seems to help clarify is to envision the follicle as a slowly-growing "water balloon" which has a tiny grain of sand inside (this sand represents the egg). When the follicle reaches about 18-20mm in diameter and a special hormonal signal, called the LH surge, is sent to it, it will undergo biochemical changes that eventually result in the popping of the follicle and the release of the egg. This special hormonal signal is what those urinary ovulation kits are designed to detect.