If the checkbox is checked, it means that a lock function is activated for each grid point. This means that each vertex that you draw snaps to the nearest point in the grid. Uncheck the checkbox if you want to place vertices in a self-selected position instead.

 

1.Turn grid on via Settings > Grid > Show grid (Ctrl-G).

2.Check the box under Settings > Grid > Snap to grid (Ctrl-L) to enable grid lock.

 

More information

See also:

Change settings - to change color of unlocked/locked grid.