Here’s How it Works
Select the objects you want to have rounded corners on and then go to “Effects” in the top tools bar, then “Effects” under “Illustrator Effects”, then click the “Rounded Corners” option
Set the Radius
You will then be presented with a dialog asking for a radius, in this case I used 20px
Need to Edit the Radius?
If you ever need to edit this radius later on you can do so by using the Appearance Panel