In years past, OSI was a protocol stack. Real software.
These days it’s a logical model, which is used to explain how different parts of the network communicate with each other.
By following this model, developers can create protocols and technologies that work with other layers, regardless of the particular technology.
The OSI model also shows how different protocols work together. For example, how the Ethernet protocol interacts with IP, and how that interacts with TCP.
In practice though, the OSI model is used mostly for teaching and learning.
There are seven layers of the OSI model:
The first four layers are the most important for network engineers, with the last three being most important to software people.