Non-reproduction blue (or non-photo blue) is a specific blue ink that is not detected by high fidelity scanners. This allows your writing (assuming you are writing in black ink, as is recommended in lab notebooks) to be easily isolated from the background grid. Many modern scanners and cameras are getting better at detecting this specific blue, but using basic Photoeditors and selecting for that color, or color channel, allows for the same quick, consistent isolation of your work from the background grid.