Filament grinding is almost always related to a clogged hotend. This is when the extruder is attempting to push the filament into the hotend but instead of the filament moving, it sits in the same spot and the teeth of the hobbed pulley shave away the filament.
To learn how to un-clog your hotend, read the instructions written for “Under Extrusion” on this page.
This can also be the result of not pushing your filament all the way into the hotend (when you thought you did) because you couldn’t push it past the tube anymore. This usually happens if you do not cut the tip of your filament sharp enough on both sides to create the shape of a “cone”. Therefore the tip of the filament may get stuck at the seam between two parts because the tip is too sharp on one side. This is why it’s best to cut it in the shape of a “cone”.