A rate limit is the maximum number of API requests accepted per second (or minute) per key; above it the API answers 429 with a Retry-After header, and batch endpoints are the way to send volume.
Limits protect the platform and each account from runaway loops. They are expressed per key over a sliding window, signalled by headers that show the remaining budget, and enforced with a 429 status that tells the client when to try again. A well-behaved client honours Retry-After and uses batch calls rather than parallel single requests for volume.
Rate limits are about requests, not messages: one batch of a thousand messages counts as one request.
At SMSMeteor: 50 requests per second per key, with X-RateLimit-Limit, X-RateLimit-Remaining and Retry-After; batches accept 1,000 messages per API request.