These functions allow PHP to interact with Mongo database servers.
MongoDB is a high-performance, open source, schema-free document database designed for cloud computing. The project's goal is a cloud-scale data store that's easy to deploy, manage and use. It's network accessible and written in C++.
More information and downloads for Mongo can be found at http://www.mongodb.org/.
There are also tutorials at http://www.mongodb.org/display/DOCS/PHP+Language+Center.