I see in masjids of Sufis they have a function say of poems and naats and at the bottom of the flyer it says "Langar will be served". What is that? Is it something bid'ah? This Langar appears on masjids of Indo-Pak communities so it could be a non Arabic word.
In mainstream culture, a langar is normally associated with the religion of Sikhism . As per wikipedia, it is a community meal from the Sikh community kitchen in a Gurdwara where a free meal is served to all the visitors, without distinction of religion, caste, gender, economic status or ethnicity. The free meal is always vegetarian. People sit on the floor, eat together, and the kitchen is maintained and serviced by (mostly) Sikh community volunteers.
In the Indian sub continent, both Sikhs and the "Sufi" muslims use the same term for their community meals. Among the Sufis, the food is served out of a massive pot called a deg often in the precincts of a dargah (Sufi shrine), and is also usually vegetarian. It is actively distributed to the poor.
Serving the community free meals by itself can't be considered bidah but an act of sawaab. Many Prophets and Sahaba are known to have fed the poor and helped the needy.
What is of concern to the non-sufi ulema are the rituals that have crept into the langars due to the proximity of the shrines (dargahs). But I won't digress as that is a separate topic in itself best left to a separate question.