The Udasis are an ascetic order sitting on a boundary that Sikhism draws firmly, and their position illustrates the boundary better than any statement of doctrine would.
Sikhism is emphatically a householder religion. Guru Nanak rejected renunciation, celibacy and withdrawal, and taught that the religious life is lived in work, family and community. The Udasi order is traced to Sri Chand, Nanak's elder son, who took up precisely the ascetic life his father had declined to recommend. Whether that traditional account is exact is less certain than the use made of it: the order becomes prominent as an order in the seventeenth century.
Udasis are renouncers. They are celibate, maintain their own centres, akharas and deras, and present in the manner of Indian sadhus, with matted hair, ash, ochre, and sometimes nakedness. They revere the Guru Granth Sahib, and they also honour the Vedas and Puranas, use images, and read Sikh scripture within a broadly Vedantic frame. That combination is why their position is contested, and it is why they were at home in what is sometimes called Sanatan Sikhism, the older and more accommodating understanding of the tradition that the reform movements displaced.
Their historical service was substantial. Through the eighteenth century, when the Khalsa was under armed persecution and Sikhs were periodically outlawed, Udasi mahants held and maintained many of the shrines, and carried the teaching into regions the Khalsa did not reach.