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.
A heart rate that's too slow is called bradycardia. What's too slow depends on a person's age and activity. A newborn usually won't have a heart rate of less than 80 beats a minute. An athletically trained teenager may have a normal resting heart rate of 50 beats a minute.