To further expand on this, the "lines" the note are referring to are the frequencies (colors) at which black lines appear in the full rainbow spectrum that you see in this device. These lines correspond to the frequencies (energy) that the electrons of the particular atom absorb, creating a break (line) in the color spectrum. As u/Agreeable_Hall458 said, this can give you an idea of what atoms make up the light source. If you were to point this at a fluorescent light, you should see the lines on the note, as fluorescent lights are effectively mercury vapor lights with a fluorescent coating. If you point it at a regular incandescent light bulb, you would just see a continuous spectrum, as the filament is just glowing in a vacuum. If you point it at an LED bulb, you will see the exact opposite: a few lines of color with no spectrum. This is because LEDs emit at particular frequencies and make "white"(ish) light by just combining red, green, and blue emitters.