You can make content available to you when you are offline. To do this, click the little cloud icon at the top right of the window that opens when you select a meditation (make sure you have an internet connection when you're doing this!). This will download the meditation to your device for you to listen to when you're offline.
All downloaded content will remain in the same location. For example, if you have downloaded Breath and Sound from the Mindfulness Foundations program, to access it head to Explore > Adults > Mindfulness Foundations > Breath and Sound. When you have no internet connection you will only see the meditations you have available offline.