Lets say the JACKPOT number is "6" (simplified). If you e.g. have a 1 in your hand and the dealer offers you a 9, the dealer says "better", because the 9 is only 3 off of the 6, not 5 off. (9-6 = 3 < 5 = 6-1).
(And you can deduce the JACKPOT is at least a 6 or higher, because if it was a 5 then 9 would be 4 off and the 1 would also be 4 off => it would not have said "better".)
You only have a limited amount of cards in the deck, so you have to balance risk vs reward of pulling more cards to calculate the jackpot or gambling and hoping that you might hit it.
Is there anything else I can explain?
(and write your highscore into the comments :)