This Sekonic website has a good interactive example of how various percentages of fill flash work with natural light.
Why would you want to use flash outside with plenty of sunlight? Bright sun is often too contrasty for digital photography. If you expose the lighter areas properly for the sunlight, shadows are often too black. The trick is to use your flash at less than full power to open up the shadow areas and add a catch light to the eyes. In the Sekonic example, I like the 30 percent picture. Your taste might differ.
So how does it work. A weak fill flash contributes a higher proportion of light to the shadow areas than to the highlight areas. Let me give a number example. Suppose the sunlight is 100 units of light and the shadow area is 1 unit of light (this is not a realistic difference but the math is easy).
Further, suppose your weak fill flash is 1 unit of light.
When you make the picture, the flash contributes 1 unit of light to the 100 of sunlight to equal 101 units of light. That is a 1 percent increase.
But the 1 unit of flash added to 1 unit of light in the shadow area is 2 units or a 100 percent increase.
That is why fill flash is so effective in opening up shadows. It makes a negligible contribution to highlights but a very good contribution to the shadows. The trick is getting the proper balance for the look of the picture you are making.
Many modern cameras have a separate control on the camera for reducing the output of the flash. Start experimenting with a -2/3rds or a -1 setting as a compensation for your auto flash exposure.
It is always helpful to read the manual of both your camera and your flash unit. Knowing your equipment leads to better pictures.