The bankroll is the total amount that players have invested for an specific currency. Cryptosdice uses 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 and adjust the kelly % to own more or less % of the bankroll. Let's say that the bankroll is 90 BTC with an average kelly of 1%. This means that the maximum win in a single bet is 90 * 0.01 = 0.9 BTC. Then you invest 10 BTC at 1% kelly, now the bankroll is 100 BTC and you have 10% stake of the bankroll, so you earn 10% of any profits. You could also had invested 1 BTC at 10% kelly and this also gives you a 10% of the profits.
You can add or withdraw funds from any investment. You can customize the kelly % and the blocked days for each investment. If you block your investment for a certain number of days, you will not be able to add more lock days or to withdraw funds, but you still can add funds to that investment.