The phrase tin top refers to a type of counter or table that was common in the 1950s. In essence it's a plain old table with a thin sheet of metal covering the tabletop (usually made of out steel, not tin as the name would suggest) coated in enamel. Known for being both indestructible and ugly, it's typical of 1950s engineering, designed before planned obsolescence, and before people shopped for things soley based on appearance.
A tin top can also refer to a diner or lunch counter so named for containing a tin top counter, usually with a row of stools for patrons to sit at and eat their lunch.