Parallel processing is a system implementation approach that involves running the new system and the old system simultaneously for a period of time until the new system is verified and accepted. The primary advantage of parallel processing is that it provides assurance that the new system meets performance requirements and produces the same or better results as the old system. Parallel processing also minimizes the risk of system failure and data loss, as the old system can be used as a backup or fallback option in case of any problems with the new system.