The phrase pied-à-terre literally translates from the French as "foot on the ground". This is said to be because pieds-à-terre were traditionally ground-floor apartments; however, more likely, the meaning of the phrase is "to have a foothold"; terre (ground, soil) referring to territory, rather than elevation.
A pied-à-terre is a residence that will be used temporarily, mostly as a second home. It is common in New York City realty for many building co-ops to prohibit pieds-à-terre because they only want buyers who will actually make the building/apartment their permanent place of residence.