Rate limiting restricts the number of messages sent during a period of time to avoid overloading servers.
Rate limiting is the process of restricting the number of messages sent in a period of time to avoid overwhelming responses or to a particular person who might be in multiple segments. If you have a mega deal to announce and a lot of subscribers, a single push notification could be enough to overload your servers with traffic and put a major damper on your momentum. Better to set rate limits so that, say, only 10,000 messages send each minute, which will stagger the response.
If using rate limiting, it's important to do the math for any time or supply limited offers. Depending on your database, limits might mean that some messages won't be sent for a day or longer (5M at 10K per minute will take more than two days and the offer may be expired.) You also might want to coordinate with other delivery features, like send-time optimization or frequency caps.
“Rate limiting for a popular campaign can keep your servers running and conversions moving.”