U-Boot is an open-source primary boot loader. It can be used as first and/or second stage boot loader in an embedded environment. In the context of OpenSBI, U-Boot can be specified as a payload to the OpenSBI firmware, becoming the boot stage following the OpenSBI firmware execution.
Please refer to the U-Boot build documentation for detailed instructions on how to build U-Boot image and boot high level operating systems from U-Boot prompt.