The InDesign plug‑in modules are software programs developed by Adobe Systems, and by other software developers working in conjunction with Adobe, to add features to Adobe software. A number of importing, exporting, automation, and special-effects plug‑ins come with your program and are automatically installed in the Plug‑Ins folder. In fact, most of the features you see in InDesign are provided by plug‑ins.
Once installed, plug‑in modules appear as options on menus, in dialog boxes, or in panels.