The bankroll is the total amount that players have invested for an specific currency. We have a 1% house edge and investors get 50% of the house wins or losses. This means a 0.5% Expected Value (EV) that is shared by the investors.
You can invest any amount into the bankroll. Let's say that the bankroll is 90 XLM. Then you invest 10 XLM, now the bankroll is 100 XLM and you have 10% stake of the bankroll, so you will earn 10% of the profits.