0.4% of total bets will go into a bonus pot. In each round, there is a small chance (that round's total bet value / 1000000) of the roulette wheel landing on a special "Bonus Pot" icon, pictured above. When that happens, the bonus pot will be split proportionally between all eligible players in that round.
The amount you earn from the bonus pot depends on your bet amount that round. For example, if the bonus pot hits and the total bet value that round is 500m and your bet value that round is 250m, you will earn 50% of the bonus pot.