The Shop floor literally is the floor of a factory (machine shop) where people work on machines, or the space in a retail establishment where goods are sold to customers. It is opposed to offices and so forth providing accommodation to the business's management.
In the UK, a worker "on the shop floor" may be used metaphorically sometimes as a synonym for the working class in economic classes C/D/E. In that way it may have undertones of class prejudice, but it may just be meant for an average person, the man on the Clapham Omnibus.