By merging two identical items on the field + you will get a higher-level item  .
  

Use items marked with a green lightning bolt  (Generators) to add new items to the field and merge items until you get the one you want. If the field already has the item needed for the task, its cell will be colored green.

An item that has reached the maximum level can no longer be merged with other items.

Once merged with each other, some items turn into Generators, which you will use to create new items.