Categories are a simple way to group specific achievements, and each category will generate its own tab in the advancements window!
These are added/edited in the category.yml
file in your MythicAchievements plugin folder.
Categories have their own tabs in the advancement menu, and also display as the first "node" in the category.
Example
SPECIAL:
Display: 'Special'
Background: 'textures/block/quartz_pillar.png'
Icon:
Material: NETHER_STAR