Collect a total of 100 Magic Orbs throughout the course of the game. The game will keep track of how many orbs that you've collected. As soon as you've collected 100, the achievement will unlock.
The magic orbs are red. Orbs can be found by exploring (towns and dungeons) and by looking in Secret Rooms. They're located on the ground. Just walk up to them and examine them to pick them up. In some instances (like in secret rooms), the orbs will automatically be collected, or can be walked through to collect them.
There are more than 100 of each orb in the game, so there is room for error - but collecting the orbs as soon as possible is recommended.