Wikipedia to the rescue:
"PostScript Printer Description (PPD) files are created by vendors to describe the entire set of features and capabilities available for their PostScript printers.
A PPD also contains the PostScript code (commands) used to invoke features for the print job. As such, PPDs function as drivers for all PostScript printers, by providing a unified interface for the printer's capabilities and features."
Do a Google search to learn more.
In the context of InDesign, PPD files are used to “table drive” the creation of PostScript for specific printer models when printing ot PostScript printers. Device resolution, available paper sizes, imageable areas, input trays, output trays, duplexing and collation capabilities, etc. are all driven by the contents of a PPD file.
In the general case, both for Windows and MacOS, once you have installed a printer driver for a PostScript printer, the PPD file you need for printing from InDesign is already installed and available not only for the driver, but also for InDesign, Illustrator, Photoshop, and Acrobat which also reference the PPD file directly.
Note that when creating PDF, no PPD file is required for PDF export. Note that PDF should be as device-independent as possible, the totally opposite concept from that of PPD files. (That is why in the past, when creating PDF via distillation of PostScript was fairly prevelant, you used the PPD file Acrobat Distiller, not the PPD for the device to which the PDF file would ultimately be printed!)