Vimshottari dasha divides a life into nine planetary chapters totalling 120 years: Ketu 7, Venus 20, Sun 6, Moon 10, Mars 7, Rahu 18, Jupiter 16, Saturn 19 and Mercury 17. Which chapter you were born into depends on the moon's nakshatra at birth, and how far through it the moon had travelled sets how much of that first period remained.
Each mahadasha subdivides into antardashas of every planet in the same proportions, and those again into pratyantardashas — which is how astrologers narrow timing from decades to months.
A dasha change does not flip life like a switch; it shifts which chart promises get delivery. A strong, well-placed dasha lord tends to bring its houses' themes forward; an afflicted one brings its lessons. The chart holds the promise, the dasha sets the schedule.
See your own timeline with the free Dasha Calculator on Astrojoshya — your current mahadasha and antardasha with exact dates.