The OSI (Open Systems Interconnection) Model is a framework for standardising the functions of networking equipment.
The goal for this is to make said equipment compatible with each other no matter the internal build-up.
The OSI model was developed in the late 70’s. Basically to ‘stabilise’ the overgrowing amount of networking standards at the time.
The OSI model is going to be your tool for troubleshooting and understanding networks. The OSI model always has to go up or down in order.
Example: If you would go to ‘YouTube.com‘ your browser will handle layer 7 to 5 (taking into account that Cisco handles this as one layers themselves). The browser, operating system and it’s firewall will take care of the ports used and allowed, thus layer 4.
It’ll than pass the router which routes based on destination IP, which is layer 3. If a switch is in between, this device will take care of layer 2.
All the equipment, cables and the electrical signal belong to layer 1, the physical layer.
In the end. Layer 5 to 7 can be seen as a single one for networking specifically.