The Master List is accessed via the menu options (the location of the menu varies across devices).
The master list should be used to maintain the inventory of grocery items that you purchase on a recurring basis. This list of items are used to create the initial grocery list when you click "Start this week's grocery list".
In addition to adding new items, you can click on an item to edit it. Clicking the "Add" or clicking an item brings you to the Add or Edit Master Item which is very similar to the Add or Edit Item screen.
The only difference between the Add or Edit Master Item and the Add or Edit Grocery Item is that instead of having a "Save to Master List option", you have a "Select by default" option when working with an item in the master list. Checking this option causes the item to automatically be included in the initial grocery list created when you use the "Start this week's grocery list" feature.
Note that it is not strictly necessary to edit the master list directly to build the master list. Instead, as you add or edit an item in the current week's grocery list, you can choose to "Save to Master List" and automatically build up your master list this way.