The Corner Tool is used for rounding the corners of paths. One caveat with Vector Inks Corner Tool is once the corner is rounded, there’s no going back (aside from pressing undo of course). This is a limitation we are working to overcome and currently have a new Corner Tool with this fix in beta. But for the time being, you can being using the Corner Tool and selecting a path on the canvas and activating the Corner Tool from the toolbar.
When activated you’ll notice handles appear on the sharp corners of the path. Select the desired handles individually and when ready, drag downward to round the corners more and upward to round the corners less. Upon release, the rounded corners will be applied.