A PSD is a mockup presentation of a web page . It's like a screenshot of a page in which you can cliet business type and their working buttons or components. Now a day psd also need to reprsent a mock up for how the page would look on a phone versus a laptop.
We make layers named by each desigers that helps to keep in mind that the PSD is a guideline to follow but not the only representation. Users have different screen sizes, different types of devices, and different interaction states. As a developer, you need to weave all of these aspects together into the web page in the form of responsive design and persistence of user state.