This is just the built program in a ready-to-use state. You can launch the application by running SpaceTrimmer.jar on a system with Java redistributable installed. If it doesn't instantly launch for you, then use the advice listed here: https://stackoverflow.com/questions/19966394/how-to-open-run-jar-file-double-click-not-working
My own take on running it in case of problems (Windows tutorial only):
- move SpaceTrimmer.jar and to \Program Files\Java\jre (yourversion)\bin
- create new .bat file containing: "java -jar SpaceTrimmer.jar" and name it as RunTrimmer.bat
alternatively, move cmd there or navigate cmd to that folder and write 'java -jar SpaceTrimmer.jar' in prompt.