This tutorial explains how to use the Fillet and Chamfer tools in Onshape. The Fillet tool allows you to create rounded corners or edges on a 3D shape whereas the Chamfer tool allows you to create sloped or angled corners or edges (at a 45 degree angle, by default).
Here is an example of a cube with a Fillet edge:
And here is an example of a cube with a Chamfer edge:
Watch the video below to learn how to apply a Fillet or Chamfer to shapes in Onshape.