Rate limit
A cap on how many requests you can make to a service in a given time window — enforced to protect the service from overload and to give predictable capacity to all users.
A rate limit is the maximum number of requests you can make to a service per unit time — per second, per minute, per day. Every API rate-limits; most consumer products do too, whether they say so or not.
Rate limits exist for two reasons. First, capacity planning: shared infrastructure has finite compute and memory, and one heavy user shouldn't degrade experience for everyone else. Second, abuse prevention: without limits, one bad actor could overwhelm a service with automated traffic. Both concerns are legitimate.
When you hit a rate limit, the service returns an error (usually HTTP 429 "Too Many Requests") with information about when you can retry. Well-designed systems queue and retry automatically; poorly-designed ones fail the whole operation. For consumer chat products, rate limits usually surface as "you've reached your usage cap, try again in [time]".