The Microsoft OneDrive icons in desktop notification areas, menu bars, Windows File Explorer and macOS Finder tell you the sync state of the file or folder.
You can learn about these icons and what they mean here.
If you don't see any desktop notification or menu bar icons, or the OneDrive icon in the Windows icon tray, OneDrive may be hidden, not installed or not running. You can relaunch OneDrive from the Start Menu on Windows or from Launchpad on Mac OS.