L refers to the live line; n refers to the zero line.
Zero line live line refers specifically to the power supply line of civil power. The AC power supply voltage of municipal power is 220 V (different countries, China is 220 V). It includes a zero line [n] and a live line [l]. The ground wire (ground is zero, PE) is called zero line.
In China, red is commonly used to express L (Live) line, i.e. live line; blue represents n (neutral) line, i.e. zero line; yellow and green (commonly known as fancy line) represents ground line (e line).