This number shows the temperature in the first location you have in the list of your Weather Live copy. As soon as the temperature changes the number should change correspondingly. This way you can check the temperature without even opening the app.
So how do you set the temperature to be displayed on the icon badge?
For this, open the device's settings.
Scroll the screen down, tap the app, and switch the toggle next to the Show temperature on badge to ON.
The red number with the temperature will appear on the icon badge.
If you feel this feature is bothering you, repeat the steps described above and switch the toggle to OFF.