that's the average of all the reviews. If you look further down on the page of the product you'll see a breakdown of how many people gave it 5, 4, 3, 2, or 1 star.
for example--
https://www.amazon.com/Sonic-Alert-SB1000SS-Alarm-Clock/dp/B000EX9K40
Scroll to the bottom til you see Customer Reviews. There's. a bar graph for each rating level. This one the 1 star rating is at 40% and the 5 star one is at 36%. Interestingly on other Sonic Boom clocks like this one on Amazon has different numbers for the ratings.
I check reviews, I don't always believe them but it's nice to know what to watch out for and also a good way to get answers to questions I may have.