Long polling is a process where browser sends a request to server. Server holds that request and keep the request thread open until any data is available. Once data is available at server, it sends the response to the browser and closes request thread. Once browser receive the response, it initiates another request and the process continues. This is called long polling in web development.
MongoDB is famous NoSQL database. NoSQL database stores data in JSON format in text file or similar. These are very famous now a days and used widely because reading data from a NoSQL database is much much faster than a RDBMS like SQL Server, MySQL or other.