An expression usually taken as meaning "socially inferior", and derived from the medieval manner of sharing a meal. In wealthier households, the lord and his lady (master and mistress) were seated at the head of a long table, and the household servants at the other. The more important you were (for instance, honoured guests and family), the nearer to the top end you would sit. The salt cellar would have been placed in the middle of the table, hence the origins of the expression. There was a fine social distinction here, people "knew their place" and few people dared or wished to upset the status quo.
As a sideline, the finer cuts of meat were also served to the top end, those "below the salt" eating dishes of offal (umbles), hence "humble pie".
It is also an album by the British folk-rock band Steeleye Span, released in 1972, in their third incarnation of Peter Knight, Maddy Prior, Bob Johnson, Tim Hart and Rick Kemp. The album is their usual mix of "traditional" folk songs and modern timing and instruments, but released before they began using hard studio techniques and electronics. A good, romping, stomping listen, and a good introduction to the band, with their later Now We Are Six release a close second.