If something's unparalleled, there's never been anything like it — such as a brilliant musical performance or a record-breaking time for an athlete.
Remember parallel lines from your old geometry classes — lines running alongside each other always at the same distance that never meet and are essentially identical? Well, that's the idea unparalleled comes from — except the opposite (thus the "un"). Sporting achievements are particularly prone to being described as unparalleled, since athletes are always out to break records.
