Rate limit is a technology used in DDoS mitigation. The rate limit technology ensures that each individual asset will not make too many transactions to the protected server or network. For example, each IP cannot make more than five HTTP requests per second.
Rate limit is effective in keeping the service safe from many variations of DDoS. However, it is not considered a first line of defense because it can cause false positives. In a certain situation such as a web API rest, it may even be the first line of defense.