The Danish word indsidder means tenant.
An indsidder had his own household and did not necessarily work for the family with whom he resided. Another word for this kind of tenant is inderste. The two words have a slightly different meaning but were often used interchangeably. An indsidder typically rented a room at a farm or in a house, whereas an inderste rented a separate house at the land of a farm or another house. An indsidder or inderste could be an unwed man, an unwed woman with or without children, a widowed man or woman with or without children, or a married man heading a household with his wife and children. A tenant might be a relative of the family, which resided at the farm or house, where the tenant rented a room or a house, so it is worth paying attention to tenants at an ancestor's farm.
The words are not used anymore. The word indsidder is often misinterpreted by Danish-speaking genealogists because it seems to be related to the term at sidde inde. However, at sidde inde means to be imprisoned. Indsiddere and inderster were a part of the lowest class of the rural society but they were not prisoners.
Indsiddere and inderster were exempt from most taxes because they were too poor to pay, so there are few sources about them. Indersteskat was a tax paid by indsiddere and inderster, so if tax records are extant for the area of interest, they can be helpful for genealogist.
The image at the top: Carl V. Meyer, "En sulten ved Døren" [a hungry (man) at the door], painting, 1910; image copy, Nationalmuseet, Nationalmuseets samlinger (https://samlinger.natmus.dk/dnt/asset/24770), license: CC-BY-SA.