the levels of a multi-story building are numbered sequentially, from "one" or "ground" upwards. In some countries, the number 13 is considered unlucky and building owners will sometimes purposely omit the thirteenth floor. Even landlords who are not themselves superstitious realize that the rentability of suites on the 13th floor might be compromised because of superstitious tenants, or tenants who are afraid their own customers might be superstitious.
How it is done
Simply being skipped
Most commonly, it is skipped altogether. Although the thirteenth floor has been skipped in terms of numbering (i.e., for hotel/apartment/suite numbers), the floor numbered 14 is technically the thirteenth floor of the building; it is simply not numbered as such. Any calculations involving the height of a building based on the height of a floor should take this into account (particularly in reference to BASE jumping).