Resources and Pressure
Different planetary resources come with different types and amounts of Pressure. Pressure represents the raw materials needed to produce one of the Income Resources.
For example, produces 3 Money Pressure, causing the civilian population on the planet it is exported to try to build 3 Markets, which grant you more income for your budget, as long as the planet has enough Pressure Capacity and free tiles to do so. Electronics
produces 16 Research Pressure, causing the civilian populaiton on the planet it is exported to try to build 16 Universities, which generate research for your empire. Exotic Matter
Planets only have a limited amount of capacity and surface space to utilize pressure with.
If the resources you import provide more pressure than your Pressure Capacity, the planet is overpressured and your civilians will build less efficient buildings, wasting pressure.
Similarly, if there are no more empty Developed (Grown for Verdant races) tiles on the planet, either because the population hasn't had time to develop the tiles or because there are no tiles left to develop, the pressure will again be used up by smaller, less efficient buildings.