Resource planets generate resources.
You can't create resource planets, so make sure you start near them or expand toward them.
In the top right part of your screen you can see how many resources you have. In this case 20 and is gathering 4 more per minute.
To see how many resources a planet generates, select the planet and click on upgrades:
Then scroll down to "Resources per cycle":
Here says that is creating 2 resources per minute. Click on the black part where says "Upgrade" to generate more resources per minute.
Originally published December 14, 2018, updated December 02, 2019