Fluent API is a way of implementing object oriented API in a way that it provides readable code. A fluent interface is normally implemented by using method chaining to relay the instruction context of a subsequent call (Wikipedia:Fluent Interface Design) Ultimately, each line of code does a task oriented operation.
The Fluent API is built on top of the standard API. It is another layer which can be described as a wrapper of the all manager classes that a developer would usually use in order to create content, pages, classification units or users. Its main purpose is to simplify the development.