In short The scheduled release of previously locked tokens (team, investors, ecosystem funds) into circulation — large unlocks can pressure price.
Schedules are published in advance, so the event itself is rarely a surprise — what moves price is the size relative to daily volume and the cost basis of whoever is unlocking. A cliff that releases several months of supply in one day is a different animal from a smooth daily drip. Early investors sitting on a large gain behave differently from a team unlocking at a loss, so read the cap table alongside the calendar.