Collecting and spending gems are a way to unlock the advanced building features in the app. There are multiple ways to collect gems and spend gems, you can learn about all of them here.
The best way to earn gems is by purchasing the Bloxels Game Builder Starter Kit. You'll be prompted at various places the the Bloxels Builder App to verify your gameboard, and each time you visit the editor.
For information on unlocking your gameboard, earning and spending gems, visit the help article here.