In Germany (and countries with similar systems) the route to becoming a professor requiresrequires* an additional qualification above your doctorate. This process is known as Habilitation. So a "Habilitationsstelle" is the position / job that you will do while working towards this even higher qualification. Same as a Doktorstelle is the position / job you will hold while working towards your doctorate. So yes it is postdoctoral, but not really equivalent to the way the US or UK systems work.
* Thanks to the commenters who have clarified it's not strictly required, it's also possible to have your body of work judged to be equivalent to having completed a Habilitation.