"OP-TEE is an open source project which contains a full implementation to make up a complete Trusted Execution Environment. The project has roots in a proprietary solution, initially created by ST-Ericsson and then owned and maintained by STMicroelectronics. In 2014, Linaro started working with STMicroelectronics to transform the proprietary TEE solution into an open source TEE solution instead.
In September 2015, the ownership was transferred to Linaro. Today it is one of the key security projects in Linaro, with several of Linaro’s members supporting and using it."
You can install and run OP-TEE on some platforms as part of the reference software stacks shipped with the Arm Reference Platforms deliverables, and we have a tutorial here for running an OP-TEE Trusted Application on Juno.
Alternatively, follow the official build instructions.