Jackpot is a power-up you unlock by obtaining Blitz Cred. Jackpot uses your Overdrive meter, so hit white notes to fill it up. Press X to trigger jackpot mode when it is available. You will know when Jackpot is available for use when the Jackpot symbol (spade/diamond/club/heart) appears over your vehicle. You will notice that it is active when there is a jackpot score above your vehicle that counts up as you hit notes.
During jackpot mode, your Overdrive meter begins to drain, and the score gain is tripled until it expires. You must full combo while Jackpot mode is active in order to retain the points. If you miss a note, you lose your bonus and it will fly off screen to the left. If you full combo for the duration of Jackpot mode, the jackpot bonus will fly to the right and be added to your score.
In order to get this achievement you must score at least 10,000 points with a single jackpot bonus. This is not a difficult score to obtain for players familiar with rhythm games, but here are some tips for anybody who needs them.
First, Jackpot's triple multiplier stacks with Super Instruments. An easy way to get this would be to use an appropriate Super Instrument and Jackpot on an easy song and/or one that you are familiar with. I got this achievement on "Pumped Up Kicks", a song included with Rock Band Blitz, using Super Vocals and Jackpot.
Second, build up your track multipliers and horde Overdrive meter until you come to a slower, easy section that you feel comfortable handling, then trigger Jackpot mode. Even a really simple section of a track becomes an insane point boost above 4x, under the influence of Jackpot and Super Instruments.
Third, try not to switch tracks during Jackpot mode unless you absolutely know what you are doing. It's easy to prematurely end Jackpot mode with a poorly timed switch, so don't risk it if you are a struggling player who is just going for the achievement.
When this achievement unlocks, you should receive an immediate achievement confirmation after it happens, in the middle of the song.