The rate limit is imposed by Twitter. All third party Twitter apps (basically any Twitter app that isn't the native Twitter app) are limited in the amount of data calls they can make to Twitter's API.
Third party apps are not allowed to refresh a timeline more than 15 times in 15 minutes, are not able to post more than 15 tweets in 15 minutes, etc.
What is Twitter's Rate Limit?
Twitter enforces a rate limit for third party apps which restricts the number of Twitter API requests that can be performed within the app for each user account. An API request can be thought of as a single Twitter operation, such as refreshing a timeline or sending a tweet.
With Twitter's latest 1.1 API, the rate limit varies based upon the type of operation being performed, and resets every 15 minutes. For example, Twitter now permits 15 home timeline refreshes every 15 minutes. If you're interested in the technical details about Twitter's rate limiting see this article on Twitter's site:
What are your current update settings?