The basic idea that food and education see no barrier of the cast, religion or creed whatsoever was somewhat missing in the 15th century. During that time, caste discrimination was a major part of social life. Upper and lower castes lived in separate colonies, and there were restrictions on where each caste could live, use water wells, and access streets. People were even expected to work in the occupation of their caste and were not allowed to choose their profession freely. Let alone, the upper castes had restrictions on what food and drink they could accept from lower castes and this made Guru Nanak, the first Sikh Guru and the founder of Sikhism think about the problem of discrimination even in a basic necessity like food.
This discrimination caught his attention when he was promoting Sikhism in the Kartarpur region, which is in present-day Pakistan. He saw how the people from different castes and communities stayed separated even while eating together. He decided to break this barrier believing in the concept of oneness. So he decided to do something where there was no line between the rich and the poor, the lower cast and the middle class. He started organising Langars - a community kitchen that serves free meals to anyone regardless of caste, religion, or social status.
This concept not only had one purpose but also touched upon another aspect of Sikhism. Besides the virtue of sharing, Langar also teaches equality and loving kindness. After paying a visit in the gurudwara to the Guru Granth Sahib, the devotees or the Guru’s guests sit on floors together in lines called Pangat and are served with Langar. The food is relatively kept simple so that no upper class can turn it into a feast and consider it a regular service which is for all. Generally, you will find roti, one kind of sabzi, chawal (rice), Langar ki dal which is everyone's favourite and a sweet like kheer sometimes in the meal. The menu almost remains the same wherever you go in India or the world over following the teaching of Guru Nanak.
There is more to it - other than oneness, the part of serving and preparing the meal also plays a huge role. Preparing and serving in a Langar works on the principles of unity and ‘Seva’ (service). Especially on occasions like Gurpurab and Baisakhi, families gather at their local gurudwaras and begin the preparations. The widespread of this simple concept became so large with time that Gurudhwaras abroad started adopting it. What started with a simple concept of bridging the gap in the caste system is today one of the biggest community services. The Golden Temple in Amritsar, Punjab has the world's largest free kitchen or community kitchen that serves 50,000 to 100,000 hot meals every day, throughout the year.