There are 16 Mask Shards. For every four that you collect you are awarded an additional unit of health.
This straightforward achievement is unlocked after collecting four Mask Shards and receiving your first health increase.
The easiest four Mask Shards to obtain are are probably as follows:
* buy one from Sly in Dirtmouth for 150 geo
* buy another from Sly in Dirtmouth for 500 geo
* rescue 5 grubs, then talk to Grubfather
to receive one.
* use the Mantis Claw in Queen's Station
to reach one.