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.
There are three general ways to classify heart rate, 1) normal, 2) fast and 3) slow. A resting heart rate is normal between 60-100 beats per minute. A resting heart rate is fast (i.e. tachycardic) at greater than 100 beats per minute. A resting heart rate is slow (i.e. bradycardic) at less than 60 beats per minute.