It is Roll Down Time
Normally, for each week that the Euromillions jackpot is not won, all the money in the jackpot pool for that week, is rolled-over to the jackpot pool for the following draw, so it gets bigger, and bigger. But, when the jackpot reaches Jackpot Pool Cap or the sterling equivalent, the rules change, and something called a Roll down comes into action.
So each week after the Jackpot Pool cap limit is reached, the extra money that would normally be added to the jackpot for the following week, is now added to the next lower prize pool in which there is at least one winner. Now normally that would be the match 5 plus 1 lucky star prize band (but it could be the match-5 or even match 4+2, etc.).
To win the Euromillions jackpot you have to guess the five main numbers (from 1-50) and two lucky stars (numbers 1-11) and the odds to doing that are 1:116,531,800, which is a big number in anybody's book! But matching 5 numbers and one lucky star has odds of 1:6,473,989, which are much better.
So if you are ever going to play the EuroMillions lottery, the time you hear the jackpot is going to be rolled down, is the time to buy your tickets.