Hey Sam!

To win the jackpot, you need to roll 7 a certain number of times in a row (the current streak is 13 times). The Jackpot counter will let you know how many 7's you've hit in a row. If you roll a number that does not have a 7 in it, the counter will reset to zero. Although, you will automatically receive a 0.001 btc free bet for making it half way to the jackpot (ex. streak of 6).

Hope that helps to answer your question, feel free to PM me if you have more!
Calvin