The Rounded Edges feature allows you to simulate fillets on parts, without actually altering the geometry in your modelling software and thus causing an increase of triangle-count.
The adjustment sliders can be accessed by selecting one or more part-level objects in the Scene Tree, which will then display the Rounded Edges accordion in the Properties sub-tab below.
The Radius slider is unit-aware to your Scene Units. Because it is a simulated effect, you will experience the best results if you keep this value low.
The Minimum Edge Angle slider allows you to limit Rounded Edges to corners greater than the set degree value.