The dungeon has many different floors that must be cleared before progressing the next. Once you clear a floor, you will receive basic rewards for it, but it can’t be completed again.
It is the best way to obtain Chips. These items have a certain single stat on them, and can be slotted into heroes to make them more powerful.
The Sweep function will automatically obtain rewards from previous levels below your current one. It takes a few hours to complete, but you can also complete it instantly with diamonds.