Certain slots require X amount of a certain item. Here's an example:
In this case, 9 x traffic lights are required. This can be done in two ways:
1. Drag the required item towards the slot
2. Select the item-slot and a button should appear at the top looking like this:
Once you click on the green button, the game will collect all the traffic lights on your mainboard and apply them to the slot. Keep in mind the button will only appear green if you have the item.
If you do not have the item, the button will be greyed out:
