What does backing mean in traditional Chinese painting?

It means to stick it on a cushion for reinforcement or display.

According to the modern Chinese dictionary, painting mounting should be "mounting" or "mounting back". Refers to sticking to a cushion for reinforcement or display; To attach (a piece of paper, for example) to a solid material as a binding. That is, use paper or silk as a foil to decorate calligraphy and painting books, or repair them to make them beautiful and durable. Where the mounting must be two layers, and the painting and calligraphy face outward, it is called mounting; Those who set off their backs with dye-free plain paper are called raccoons.

mounting is a traditional industry in China. It has a history of more than two thousand years. Mounted products, such as separate shafts, nave, couplets, horizontal drapes, strips, lenses, sectors, hand scrolls, albums, etc. Its basic steps include supporting the bottom, repairing, supporting and drying; It needs to go through dozens of processes, such as pulp mixing, back support, wall mounting, strip adding, silk mounting, shaft mounting and label adding. Emperor Taizong attached great importance to mounting paintings, and appointed Wang Xingzhi's calligraphy and famous paintings of past dynasties to mount them directly, while Chu Suiliang and Wang Zhijing supervised them. Song Huizong established the Painting Academy, and the mounter was one of the six pending letters of Wensi Academy. Since then, China's painting and calligraphy mounting skills have been introduced to the people. Mounting skills in Ming and Qing dynasties became a specialized industry.