Generators are a type of item, indicated by a green lightning , which can generate new items if there are free cells on the field. Generators can be found by clearing the field or can be created from some regular items. Like normal items, two identical Generators can be merged with each other to create one Generator of a higher level. Generators usually generate items of the first level, less often the second.

After a generator produces a certain number of items, it needs some time to recharge.

To add a new item to the field, simply select the Generator and then click on it again. For each such action, 1 unit of energy  is spent. Note, that you can't choose which item will appear from the Generator: one of the possible generated items will simply appear on a free cell of the field.
  

To find out what items a particular Generator can generate, click on it and then on the  button on the top right. In the window that appears you will see the level of the selected Generator, as well as the items that can be generated by it.