Bitcoin is the currency of the Internet: a distributed, worldwide, decentralized digital money. Unlike traditional currencies such as dollars, bitcoins are issued and managed without any central authority whatsoever: there is no government, company, or bank in charge of Bitcoin. As such, it is more resistant to wild inflation and corrupt banks.
Links:
https://www.autoweek.com/racing/indycar/a36418398/bitcoin-indy-500-racing-team/
They call it a sponsorship, but sounds like the owner set up a fund to self, or maybe crowd source, the sponsorship. Not clear to me.