Most file-hosting sites like Rapidgator or DDownload track your IP address to limit how much you can download for free. A proxy acts as an intermediary, masking your real IP so the hoster thinks each request is coming from a different person.
A VPN is generally superior to a proxy for downloading. jdownloader free proxy
If you only want to use the proxy for a specific website (to avoid slowing down other downloads), use the "Hoster" filter in the connection settings. Most file-hosting sites like Rapidgator or DDownload track
These sites provide regularly updated lists of public proxies that can be copied into JDownloader: jdownloader free proxy