The iOS operates with the idea that you remain connected to the internet constantly, which is largely available in this modern world of 4G networks and public Wi-Fi connections.
This, however, is not always practical as we are still areas where we do not have connectivity and many people are still using iPads and iPod Touchs that are not 4G capable. Since going onto web content with Safari is a primary function for these devices, you may still want to browse some of this content when you are not connected. This is why the Reading List feature was added to the iOS, so you had the option to look at content when connectivity has failed you. Here is a look at how to use Reading List of the iOS.
If you want to add an article that you may be reading to Reading List, start by choosing the small button at the bottom of the Safari display that looks like a curved arrow coming out of a box. From the share menu hit the Add to Reading List button, which is in the lower right hand corner directly to the right of Bookmark. When you choose this it will close out the Share list and it will be saved to your Reading List.
Now that you have an item added to your Reading List, you can push the button next to the Share button that looks like a book. This is going to show your Bookmarks, including History and things in folders. At the very top will be Reading List, which you select to look at the list that has formed. Here you will see these articles broken into two tabs: All and Read. This allows you to keep track of both what you have read and the sum total of articles you have added to the Reading List.
The Reading List pages will then be accessible in situations where you get no service or have Airplane Mode activated. It does not, however, allow you to actually interact with websites in the same way that you could when connected to the internet. It is not a way to browse offline, but instead just a way to read longer content when not connected.