That error means that you have used up your API quota for the day.
You are allowed either 300 or 10,000 API requests per day for each IP address, or app, depending on whether you use an application key and/or an access_token.
See:
- The "Throttles" documentation
- What are the API request limits? (FAQ)
- Request Throttling Limits (FAQ)
Note that you must pass the key with every call to get the quota benefit.