Rate limits are restrictions placed on the frequency of requests an API can process. Rate limits are necessary to ensure server resources are not over-taxed, prevent malicious attacks, and create a great user experience for both users and application developers.
Rate limits are applied per user or IP address and can be set as absolute numbers of requests or requests per period of time. For example, an API might enforce a limit of 10 requests per second from an individual user or IP address. When the rate limit is exceeded, the API will respond with an error rather than the requested data to limit the demand on the server. Rate limits also usually increase with authenticated requests, as authentication will generally be enforced on more important data or resources.
Rate limits are important because they protect the functionality and availability of the API by ensuring that there are enough resources available to give desirable performance. Without rate limits, malicious users or automated scripts could cause an API to become overloaded with requests, resulting in poor performance and making the API unavailable to others.