Paul McCartney wrote this as Hey Jules, a song meant to comfort John Lennon's 5-year-old son Julian as his parents were getting a divorce. The change to Jude was inspired by the character Jud in the musical Oklahoma! (McCartney loves show tunes)
The medical term for a slow heart rate is bradycardia; generally defined as a heart rate below 60 bpm (beats per minute). In contrast, tachycardia means fast heart rate above 100 bpm.
Normally the heart beats 60 - 100 times per minute. The rate may drop below 60 beats per minute in people who exercise routinely or take medicines that slow the heart. If your heart rate is fast (over 100 beats per minute), this is called tachycardia. A heart rate slower than 60 is called bradycardia.