The phrase to sit below the salt means to be of lower social standing or worth.

In this phrase, salt is used for saltcellar. For example, a will written in 1493 contains “To John Wymer and Margarete his wif a cuppe and a salt of silver”. In the first edition of Dictionary of Phrase and Fable (London, 1870), the English clergyman and schoolmaster Ebenezer Cobham Brewer (1810-97) explained:

To sit above the salt—in a place of distinction. Formerly the family saler (salt-cellar*) was of massive silver, and placed in the middle of the table. Persons of distinction sat above the “saler”—i.e. between it and the head of the table. Dependents and inferior guests sat below it.

* The term saltcellar is pleonastic, since the second element is an alteration (due to association with cellar, meaning storeroom) of saler, an obsolete word meaning salt cellar and derived from the Old-French feminine noun saliere (modern French salière), from Latin salarius, meaning of, or pertaining to, salt.

In Folk-Lore of Shakespeare (New York, 1884), the Reverend Thomas Firminger Thiselton Dyer (1848-1923) wrote:

In days gone by there was but one salt-cellar on the table, which was a large piece of plate, generally much ornamented. The tables being long, the salt was commonly placed about the middle, and served as a kind of boundary to the different quality of the guests invited. Those of distinction were ranked above; the space below being assigned to the dependants, inferior relations of the master of the house, etc.

The earliest known use of the phrase, in the form to sit above the salt, is from Virgidemiarum: sixe bookes. First three bookes (London, 1597), by the bishop of Norwich, religious writer and satirist Joseph Hall (1574-1656); this is the beginning of one of the satires:

(1602 edition)
A Gentle Squire would gladly intertaine
Into his house, some trencher-Chaplaine:
Some willing man that might instruct his sons,
And that would stand to good conditions.
First that He lie vpon the Truckle-bed,
Whiles his yong maister lieth ore [= over] his hed.
Second, that he do, on no default,
Euer presume to sit aboue the salt.

In A Straunge Foot-Post, With A Packet full of strange Petitions. After a long Vacation for a good Terme (London, 1613), Anthony Dixon described the miseries of a poor scholar:

This fellow for all this shal be lodged next the kitchin, where the Cookes, and sculles keepe such a scolding that they will take order for his studying: or in some ruynous roome, where his Masters Fathers Ghost is reported to walke […]. Now for his fare, it is lightly at the cheefest Table, but he must sit vnder the Salt, that is an Axiome in such places: and before he take his seat, Memorandum he haue two legs in store, one for the Maister, another for the Mistresse: Then hauing drawne his Knife leisurably, vnfoulded his Napkin mannerly, after twice or thrice wyping his Beard (if he haue it) he may reach the Bread on his Knifes point, and fall to his porridge and betweene euery Sponefull take as much deliberation, as a Capon crāming (least he be out of his porridge before they haue buried part of their first course in their Bellyes).

In The guls horne-booke (London, 1609), the English playwright and pamphleteer Thomas Dekker (circa 1572-1632), using the term saltcellar, played on the phrase (ordinary denotes an inn where meals are provided at a fixed price):

At your twelue-penny Ordinary you may giue any Iustice of peace, or yong Knight (if hee sit but one degrée towards the Equinoctiall of the Salt-seller) leaue to pay for the wine, and hée shall not refuse it, though it be a wéeke before the receiuing of his quarters rent, which is a time albeit of good hope, yet of present necessity.

In Lanthorne and candle-light. Or, The bell-mans second nights-walke In which he brings to light, a brood of more strange villanies than ener [sic] were till this yeare discouered (London, 1609), the same author used beneath the salt:

Into an Ordinary did he most gentleman like, conuay himselfe in state.
It seemed that al who came thether, had clocks in their bellies, for they all struck into the dyning roome much about the very minute of feeding. […] In obseruing of whom and of the place he found, that […] it was a schoole where they were all fellowes of one Forme, & that a country gentleman was of as great comming as y^e proudest Iustice that sat there on y^e bench aboue him: for he that had the graine of the table with his trencher, payd no more then he that plac’d himself beneath the salt.

A figurative use of the phrase occurs in the anonymous The play of Dicke of Devonshire; a Tragi-Comedy (circa 1636):

(1905 edition)
                                        Must my elder brother
Leave me a slave to the world? & why forsooth?
Because he gott the start in my mother’s belly,
To be before me there. All younger brothers
Must sitt beneath the salt & take what dishes
The elder shoves downe to them. I do not like
This kind of service.