Bonus Cash can be used to enter tournaments and compete for real cash prizes. You cannot withdraw Bonus Cash, and your Bonus Cash is forfeited whenever you make a withdrawal.
When you enter a match, you typically pay the entry fee with 90% real cash and 10% Bonus Cash. However, if you do not have enough real cash to cover 90% of the entry fee, a larger portion of Bonus Cash will be used.
When you win, your entry fee is refunded the same way you paid in (split between real and Bonus Cash). However, the winnings on top of your entry fee are 100% real cash.
The team at Skillz provides more information about the bonus cash features here:
https://skillz.zendesk.com/hc/en-us/articles/203685769-How-does-Bonus-Cash-work-and-how-can-I-get-more-of-it-