Rate limit
A rate limit is a cap on how many requests you can send to a service in a short time.
In a simple example
A service allows 60 requests a minute. On the sixty first, it answers: Please wait.
Why it matters to you
It protects the service from overload, and it protects you from a runaway program that asks again and again.
Do not confuse with
A rate limit is about speed. A spend limit is about money.