Nichirin Blades are special blades used by and made specifically for the Demon Slaying Corps to slay Demons. They are forged from a unique ore that constantly absorbs sunlight, one of the only major weaknesses of Demons. Therefore, decapitation by Nichirin Blade is one of the only ways a human can kill a Demon, the other being the use of a Wisteria-poison coated weapon.
Nichirin Blades take on a distinct color when first drawn by its owner, which is why they are also referred to as "The Color Changing Swords". However, the holder is required to have a certain amount of skill, or the blade will not change color. Zenitsu Agatsuma, wields a Yellow Nichirin Blade, symbolizes thunder. Zenitsu may have low self-esteem and may seem like a coward, but when the situation demands it, he lets his true power show at the speed of lightning. Zenitsu's true strength shows up as unexpectedly and explosively as thunder itself.
While most Nichirin Blades take on colors as a solid band running down the length of the blade, the coloration on Zenitsu's blade is unique in that it instead forms a lightning bolt motif down the length of the blade. It isn't known why his blade took on that pattern, but it does make for an awesome looking sword.