The word langar means literally kitchen. In Sikhism it is often used in the phrase "Gur ka Langar," which refers to the Guru's kitchen. However the true meaning is closer to sacred food service, or divine dining. Langar encompasses both the cooking facility connected to the Gurdwara, and a concept of bibek, or conscious cooking while meditating on the divine to inspire humility which manifests in seva.
Langar feeds the body of the sangat, or congregation being served, and nurtures the soul of the one performing service. Recipients of langar, the diners, also engage in the act of humility by sitting on the floor side by side with out regard to rank.
Every gurdwara has a langar where all people are welcome to a free meal regardless of their sex, colour or religion. There are no rituals observed in the langar and everyone eats together. All the food is vegetarian so that no religious group is offended.
Guru Nanak Dev Ji established the langar because he rejected the Hindu caste system where people of different castes do not eat together. Guru Nanak Dev Ji wanted to stress the idea that everyone is equal. Everyone shares the tasks of preparation, cooking, serving and cleaning. This shows sewa - selfless service to the others in the sadhsangat (community), the gurdwara, and the world outside.
The teaching of the langar was continued by Guru Amar Das Ji (the third Guru) who made a rule that no one, however important, could see him until they had first eaten in the langar.
Examples from Gurbani
Freeing the mind from ego using the tongue to recite Gurbani and naam, enables one to absorb, and digest, the langar of Guru’s word.
ਲੰਗਰੁ ਚਲੈ ਗੁਰ ਸਬਦਿ ਹਰਿ ਤੋਟਿ ਨ ਆਵੀ ਖਟੀਐ ॥
"The dining hall of the Guru's word is open; its supplies never run short."(SSGS||967)