"Rate-limiting" is used to control the amount of incoming and outgoing calls to the API. It is used to protect the API from inadvertent or malicious overuse by limiting how often each of our users can call the API. Without rate limiting, a user may request as often as they like, which can lead to “spikes” of requests may "starve" other users from using the API.
Let's say you're on the Free plan that is configured to allow 1,000 requests/day. If the number of API calls you make exceeds that limit, then an error will be triggered and you will not be able to get the data until the day. The Tomorrow.io API has rate set on an hourly, daily and monthly basis (or fixed windows) for each of our plans.