Not the OP, but BSL is Source License that gives you access to use the source, with specific restrictions.

Most commonly the restrictions prevent you from launching a competing offering. In their case, you can't offer database-as-a-service using their code.

BSL typically also restricts production use - though it looks like ReadySet has relaxed that restriction.

Finally, BSL reverts to a traditional open source license after a set period of time - in their case Apache 2 after 4 years. This means that code written today is licensed under BSL for 4 years, then automatically reverts to Apahce 2 thereafter.