Facing pages in InDesign refers to the feature that allows you to display two pages side by side (just like how two pages look in a paperback book). This helps in designing spreads and making sure that the design elements look correctly when the document is printed, as the two pages have a common spine that binds them together.
You can enable the facing pages feature when creating and flipping through work by simply clicking the "Facing Pages" toggle in the Pages Panel. Once enabled, you will see a red bar which indicates the center line that acts as the spine. The design you create can then be folded at this red bar when it is printed.
You can also duplicate your spreads easily by simply dragging the spread onto the New Spread icon at the bottom of the Pages Panel. This will make sure that any spread created on one side is automatically also created on the other side.
Additionally, InDesign also allows you to set margins, bleeds and tables differently on each side. This makes sure that both pages look great when they are printed, as they are bound together in one document.
If you want to read more on the topic, you can check Adobe's link on "Working with Facing Pages" or the InDesign Basics Tutorial on "Facing Pages".
