The main idea is, to allow the player to build improvements, which count as buildings in nearby own cities.
To build such an improvement, you need to build a special unit, which is kind of a construction set for the building you want build. It will be lost after building the improvement they are made for. As long as you stay in control of the improvement, you will notice a building in the city marked with an "(e)" for "external", which can give you various bonuses.
See forum for the rest: Construction Sets Thread
Construction Sets 2016-10-05