The Inventory   allows you to free up space on the merge field by moving items into it. To move an item to the Inventory, simply drag it there. To place it back on the field, tap the item in the Inventory and it will be returned to any empty cell.

If you run out of space in the Inventory, you can purchase an additional pocket using crystals.

Please note:
If you place a Generator in the Inventory, it will not regenerate, and its timers will not progress. In addition, items stored in the Inventory do not count toward tasks - they must be placed on the merge field to be counted.