The terms FIFO and LIFO refer to inventories and how they are accounted.  These terms are acronyms.  FIFO stands for First In First Out, while LIFO means Last In First Out.   These are accounting terms and are not necessarily related to how physical goods move in and out of a warehouse.  To explain the point further, a fictitious (and highly exaggerated!) example will be provided.

Let’s say that for the last fifteen years, the company accounted for its inventory using LIFO.  That is, when it prepared its income statement, the value it assigned to the subassemblies was the price it paid for the last inventory item to enter the warehouse.  If all other expenses average out at $10 per widget, and the widgets sell for $100 each, then the profit per widget is

$100—$70—$10 = $20 per widget (under LIFO)

The company sees a slow down in sales, so the new CFO decides to switch from LIFO to FIFO—you’ll see why in a second!  With FIFO, it calculates the value for subassemblies on the first one to enter inventory.  Well, inventory has been at three months or more since 1990 (when it cost $20/widget); it has a lot of “old” subassemblies!  In this highly stylized example, the new calculation for the profit on each widget becomes

$100—$20—$10 = $70 per widget (under FIFO)

This example is a bit of an exaggeration, but it does serve to give a dramatic illustration of the difference between LIFO and FIFO; i.e., when  a switch is made from one to the other.  Switching from LIFO to FIFO usually boosts profits for a short time.  If the value is material, the fact is disclosed in footnotes.
In the example on Wal-Mart, the company switched the other way: from FIFO to the more conservative LIFO.  It’s more conservative because generally prices tend to rise over time, so the latest inventory is valued higher.   If a company is being very conservative, it will generally follow LIFO to account for its inventory.   It’s always a good idea to read those footnotes!