Definition - What does RESTful API mean?
A RESTful API is an API that conforms to the representational state transfer or REST model. RESTful APIs are sometimes easier for developers to use because they have a familiar syntax and set of protocols. As more functionality has been built into the internet, developers have talked a lot about the benefits of RESTful architecture.
Techopedia explains RESTful API
A RESTful architecture uses HTTP coding for much of its functionality. It uses the established Secure Sockets Layer (SSL) encryption for security purposes. It is also language-agnostic, in a practical sense, and very compatible with many different environments.
That is not to say that RESTful architecture does not have its limitations. One big example that developers have talked about extensively is the lack of state-based data transfer in RESTful architectures. As a result, applications must be stateless, or be supplemented with some outside resource that adds in the desired state information. Again, the simplicity enabled by RESTful architecture means it still enjoys some popular use in the developer community.