What does the option of ‘continuous’ reading mean?
'Continuous' reading means that the Apple watch will collect a heart rate reading every 5 seconds. (The default for Apple Watch is every 5 minutes). People typically enable it when they want to get a finer grained view of what their heart is doing -- e.g. if they are monitoring their heart rate, working out, or even throughout the day to see which co-workers stress them out most!
Note that because the sensor takes more frequent readings, there is an impact on battery life.