The Bolt Protocol is a lightweight (binary) messaging protocol for databases. Bolt is statement oriented with a client-server design; originally created by the team behind Neo4j.
For an in-depth overview, including transport layer details, messaging, serialization, and examples see https://boltprotocol.org/ .
Originally, Bolt was created for use in the Neo4j graph database. PostgreSQL's binary network protocol and the data interchange format MessagePack inspired Bolt.