It allows the user to create rounded edges and corners. So, the user can regard creating filleted edges as the procedure to round them by rolling a ball over the edges or corners. Additionally, when filleting an edge or corner, the user can specify a start radius and an end radius for the fillet to create a variable fillet. For references, the user can get different results when applying fillets to one edge at a time and to all edges at once because it depends on the complexity of the filleting.
Figure 1 Fillet icon of the Local group in the Geometry tab [Body Edit Mode]
Figure 2 Creation of a Fillet