Broadly speaking, in test automation, a runner (a.k.a. a test runner) is software component responsible for executing automated test cases and reporting results. Runners commonly orchestrate the process of running tests: discovering which tests to run, setting up the necessary environments, executing tests, and collecting and reporting outcomes, including passed, failed, or skipped tests and errors encountered.
Under OpenTAP, a Runner supports remote starting and control of OpenTAP sessions. These sessions can create, load, and execute test plans, modify test plan and test step properties, adjust OpenTAP settings, and provide logs and results.
OpenTAP Runners are implemented using NATS. NATS stands for Neural Autonomic Transport System, and is an open-source messaging system or message-oriented middleware, a project under CNCF (Cloud Native Computing Foundation).