I think it just means a named area of land e.g. a particular vineyard, or sub-zone of a vineyard. It’s therefore normally a smaller area of land than an AC. Fairly similar to what would be labelled ‘single vineyard’ in other countries.
In most cases I don’t think there are any other legal requirements, e.g. in terms of alcohol levels, the wine will just have to comply with whatever the rules are for the larger AC in which the lieu dit is located. For example Guigal’s ‘La Mouline’ Cote-Rotie would just have to meet the requirements for Cote-Rotie in general, as far as I understand it.
I guess scrupulous producers would use them as a way of distinguishing different cuvees or quality levels, (and less scrupulous ones just as a marketing technique).