This layer is layer one in a seven layer OSI model. This layer is responsible for the details such as electrical and mechanical connections to the network and the transmission of the binary data. This layer is used by the data link layer. It does not represent data itself, however it provides the means for data to move.
At its basic definition the physical layer represents the hardware such as computers, wires, circuit boards, modems, and routers. Examples of how the physical layer works would be one physical computer on one side of the country and another computer on the other side.
Computer A in NY wants to send a message to computer B in CA. The Computer in NY would allow the end user to log into their email account via a browser window. The computer would then allow a means for the user to type in the message via the computer keyboard, mouse, and screen. Then the user sends the message.
That message will have to travel across computer A’s router, modem, LAN then to the WAN then to computer B’s modem, router and finally to computer B’s computer once user B opens their email. Then the message will be delivered or displayed to user B via the computer screen.
So to sum up, the physical layer consists of all of the wires, keyboard keys, modems, routers, and any other “Physical” equipment that may be needed to send data or messages from one physical device to another.