The offline mode is such that new presentation or changes you make will automatically be updated when you get back online, making it possible to continue slides presentation without worrying about connection disruptions.
Google Docs offline feature works with Slides presentation as well, meaning that if you have enabled offline editing on Docs you don't have to make any changes to Slides for offline functionality. If you have not enabled offline editing on Docs, you will have to follow the steps outlined on Google Drive help.
However, the offline capabilities have some restrictions based on device used, whereas on Chrome browser or Chrome OS you can be able to view and edit presentations, while on Android or iOS you are limited to viewing documents only. The company also added that they are working to make offline spreadsheets available soon.