From A Wiki of Ice and Fire
The Sacred Law of Hospitality
The guest right is a sacred law of hospitality. When a guest, be he common born or noble, eats the food and drinks the drink off a host's table beneath the host's roof, the guest right is invoked. Bread and salt are the traditional provisions.
When invoked, neither the guest can harm his host nor the host harm his guest for the length of the guest's stay. For either to do so would be to break a sacred covenant that is believed to invoke the wrath of the Gods both old and new. Both the teachings of the Old Gods and the Faith of the Seven hold to this. Even robber lords and wreckers are bound by the ancient laws of hospitality. 
Violation of the Sacred Law of Hospitality
There are two infamous instances in the history of the Westeros when the guest right was broken: the legend of the Rat Cook of the Nightfort, and more recently the Red Wedding. The song of the Rat Cook is used to represent the repercussions that await those who violate this sacred laws of hospitality.
A lord with a bared sword across his knees is making a traditional sign that he is denying guest right.
It is sometimes customary for a host to give "guest gifts" to the departing guests when they leave the host's dwellings; this usually represents the end of the sacred guest right. In addition, visiting guests will sometimes offer their host "guest gifts" as gratitude for giving them food and shelter.
Customs: Additional information on customs and traditions of Westeros.
|“||“In the north, we hold the laws of hospitality sacred still.” ||”|
|“||“I will see the dawn, at least. …I have eaten of his bread and salt”. ||”|
|“||“Guest right don’t mean so much as it used to. Not since m’lady come back from the wedding. Some o’ them swinging down by the river figured they was guests too. ||”|
|“||“Walder Frey’s fourth wife was a Blackwood, but kinship counts for no more than guest right at the Twins. ||”|
|“||“Death and guest right….They don’t mean so much as they used to, neither one." ||”|
|“||“In the Seven Kingdoms it is considered a grave breach of hospitality to poison your guests at supper." ||”|