The application layer is the 7th in the OSI model. It provides services for an app, including preparing human communication for transmission over the data network, as well as ensuring that effective communication is feasible.
This of the OSI model as a bit like an assembly line. Each layer prepares the data for the next one. The 7th layer interacts with the application whenever the user performs any network activities like transferring files or reading emails.
Application layer protocols provide the rules for communication between applications. The protocols define the processes on either end of the communication, the types and syntax of any messages, how they are sent, the expected response and the interaction with the next layer in the OSI model.
The most well-known application layer protocols are the ones that provide for the exchange of user information. These are necessary for many of the common internet communication functions. They include protocols for email such as Simple Mail Transfer Protocol (SMTP) and for the web like HyperText Transfer Protocol (HTTP) among many others used in the application layer.