When you download The Bard's Tale for the first time, it will ask you to download either SD or HD quality graphics assets as part of the install process. You can choose whether you want SD or HD depending on your available storage space and device's performance (older and lower end phones are best with SD; newer and high-spec phones or tablets are best with HD).
You only need to do this download the first time you start the game; once installed, you will be able to play without downloading anything after that.