You have created the shopping list marking each item you need to purchase, then you are ready for going shopping.
All, means all the products you need to buy. Meanwhile you are shopping you can mark each product that goes into the trolley as if you had a highlighter, but only a click is necessary.
Pending, means the products that still are not inside the trolley. You have to look for them.
The top-left button is the current selected list, you can select a new one tapping on this button or change it on the Settings tab.
If you have in Settings the option Show prices enabled, you will see a small black-grey button with the product's price, just tap on it to change the quantity or the price and see the total amount changed.
See down the total amount purchased.
A small number on the icon, remembers you the quantity of items that still remains you.
Shake the device if you want to unmark your shopping list, for example when you're done