Resources generator is a special object that can be obtained in the Quest and can produce a limited amount of certain resources for your cafe. This object looks like a piñata with a picture of a resource which it produces above it.
You need to press on the generator, and it will be placed to your storage. You can install it as an exterior object. When it is installed, it starts producing resources. If you want to check the production status, you need to press on the piñata.
Remember that the generator can produce resources only for a certain number of times. You can see how many attempts of resources production are left on the counter above the generator when collecting the resources. When all the attempts are used up, the generator will disappear.