means The padding, springs, and fabric covering that make a chair or sofa soft and comfortable to sit on.

from From the old word 'upholder,' which meant a dealer in small goods or someone who repaired and sold furniture — built on 'uphold,' to keep up or maintain. By the time the trade specialized, an 'upholster' was the person who maintained and dressed furniture, and 'upholstery' became the craft of cushioning it. The wandering 'r' is the same kind of leftover seen in words like 'huckster.'