The menstrual cycle is the roughly monthly sequence of hormonal changes that prepares the body for a possible pregnancy. It is counted from the first day of one period to the first day of the next. A cycle is often described as ~28 days, but anything from about 21 to 35 days — with 2 to 7 days of bleeding — can be perfectly normal, and some variation between cycles is common.
It's driven mainly by estrogen and progesterone, with FSH and luteinising hormone, and moves through phases: the period, the follicular phase, ovulation, and the luteal phase. Cycles often become irregular in perimenopause. See the menstrual cycle phases.