Constructions are a special action you can do during your turn. They have the possibility to help your planet grow faster and are recommended to use during your game.
To build a constructions open your constructions panel by clicking on your planet (or view your opponent's constructions by clicking on his).
For now there are 5 types of constructions you can build. You can read the description to know what it does.
Default game rules allow you to build 5 constructions per turn.