Aha allows you to download podcasts and audio-books on your phone while on wifi and listen to them later while being offline and on the move thus not using cellular data.
To download podcasts or audiobooks, play the audiobook or the episode of the podcast on the phone. On the screen, click on the down arrow icon to start downloading.
To listen to the downloaded content, go to the Menu (triple hash) button at the top left of the screen. Select Downloads and you will see the downloaded contents listed. Choose the one you want to listen to.