Number of successful MT messages submission in an interval of time (seconds) is known as TPS (Throughput per second).
If you have a SMS Gateway capable receiving a volume of 5 million messages per second means it has the message receiving capacity of 5 million. It is message per second capacity of the SMS gateway. i.e. 5 million/per second. This showcases the scalability and capacity of the SMS gateway to handle large volume of messages within a second.