If you have a cauldron equipped in one of your Tool slots, it unlocks tactical alchemy tasks in the autobattle.
When you get an alchemy task, you can put the required items in the cauldron and it will produce one of the several possible bonuses. If you don't want to use your items to complete the task, you can wait a couple of minutes and the task will disappear on its own.
Only level 1 items can be in the task requirements.
The items are taken from your inventory all at once at the same time as you get the bonus. If you put some amount of items in the cauldron but the task expires, the items will not be taken from your inventory.