The REST (Representational State Transfer) interface in provides access to lists and libraries as a relational data service. A core concept of any RESTful interface is the resource, and in SharePoint Foundation, lists and items are represented as HTTP resources that can be addressed by remote URLs.
REST APIs (which are a type of web API) involve requests and responses, not too unlike visiting a web page. You make a request to a resource stored on a server, and the server responds with the requested information. The protocol used to transport the data is HTTP. “REST” stands for representational state transfer.
You can read more about basic operations in sharepoint using REST, here.