We are not allowed to encroach on another's property, such as by moving a boundary marker, enlarging our own land at the expense of his. This is a form of theft and it violates two prohibitions: the general prohibition against stealing and the specific prohibition against encroachment.
Other applications of this principle would include opening a store adjacent to a similar store when there is not enough business to support both, and copyright infringement.
The underlying rationale of the mitzvah is that theft is theft. Even if it involves moving a fence or a sign, it unfairly deprives another person of his rightful property and is not to be permitted.
This mitzvah applies in all times and in all places. It is discussed in the Midrash in the Sifre and is codified in the Shulchan Aruch in Choshen Mishpat 376. This mitzvah is #246 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #85 of the 194 negative mitzvos that can be observed today in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.