Is used to change the spacing of the grid points. Click on the 'i' sign to the right of the field, and you get a list of all available values.
With the 'Activate snap' toggle ticked (check marked), the cursor will jump from grid point to grid point and changing the 'Grid settings' will change the effective jump that the cursor makes. You can also use <F5> to toggle the snap state.
Tip
The Library components were made using a 2.5mm - grid. If you work with a 4mm grid and the automatic size fitting is turned off, some component pins will not fit the grid (3x 2.5mm = 7.5mm).