Split MP3 is a really basic program, sticking to exactly what its name suggests.
If you want to cut any MP3 file, simply open it with Split MP3 and you can set new start and end times and make a new, shorter file. It doesn't support drag-and-drop, so you have to open your MP3 manually, then you can play, stop and mark new start and end times form the interface.
When you're happy with your edit, you can save it in the current format, or convert it to any of the main audio formats, and a selection of bitrates. Saves and conversions are respectably fast. Split MP3 is a very basic application, and misses a few features. There are no start/end markers for the timeline, which itself is missing a waveform, so it's pretty useless. You can't add fade-in/outs either, making it a super-basic editing tool.
Split MP3 is extremely basic, but does it's job well. If you need more editing funcions and flexibility, try Audacity