In order to open a jar file you will need to download Java Runtime Environment. Once downloaded all you will need to do is double click the file this works because the file extension will then be registered to that application allowing you to open the file the same way rar files can be opened with Winrar once the extension is detected by the computer it will attempt to run it with that registered application if any is present, the application will then read the data through the way it is formatted and process the data accordingly.