What is Apex Test Execution in Salesforce?

While making Run all test call, the job is queued for test run calculations, but for deployment to production, as its synchronous and "run all test" is always "true" in background, so, test classes must have to run to validate if components going in production are valid or not. This is one of the major the control check salesforce have imposed to make sure all the production instances have valid code on cloud.