A must-drop jackpot is a progressive jackpot with a guarantee: it will be won before a stated deadline, such as the end of each day, or before it reaches a stated cap. Red Tiger's daily drop jackpots are a well-known example.
Each pot is still won at random, but the drop point is chosen within the stated range, so the jackpot cannot keep growing indefinitely. Many systems run several pots at once, for example an hourly, a daily and a larger one.
Some players think a must-drop jackpot near its limit is a good bet. Any edge from that is tiny for an individual, because the win is spread across every spin from every player on the network, and the game still has a house edge. The chance that your spin is the one that wins remains very small.