There are two ways you can use your Chromebooks offine. The first is to go in to Google Drive and enable OFFLINE MODE. The directions for this are here: CTSBasicsOfflineMode
The other way is to download webpages using the Pocket App. This will allow you to view certain webpages when you dont have wifi. This app has been installed for you. It will prompt you to make an account. If you need to access a webpage while away from wifi make sure you add it to your pocket while you still have wifi. Give the app time to download the pages and then you should be able to access it through the pocket app later.
Here is a video that explains Pocket App further: Pocket
Let us know how it goes and good luck!