Ultraviolet will tell you how much UV light there is where you are right now based on the time of day and the conditions of the atmosphere. This can help you figure out how much sunscreen to wear or whether to take a little one outside.
It also shows a graph of the amount of UV over the course of the day, and over the course of the year at the current time. These graphs are draggable so you can see how your sun exposure changes if youre staying out all day, or to compare the amount youre getting now to the amount you got during that day at the beach a month ago.
You can also enter (or drag the map) to find the amount of UV hitting any place on earth at any time. So if youre heading to Florida on vacation, you can see what youre in for.