To unlock this achievement, you have to visit the 5 biomes that can be found in the Nether. These are as follows:
The Nether Wastes
, which can be considered the "classic" Nether biome. Features an abundance of Netherrack.
The Warped Forest
. Features a lot of Warped Nylium on the floor, with Warped Fungi scattered across the biome. Easily recognisable by its blue color scheme.
The Soulsand Valley
. As the name implies, this biome is covered in nothing but Soul Sand and Soul Soil.
The Crimson Forest
. Similar to the Warped Forest, except it features an overabundance of Crimson Fungi instead. Recognisable for its blood red color scheme.
The Basalt Deltas
. Features a landscape covered in Basalt, with the occasional Blackstone dotted around. Recognisable by its grey fog and white ash floating around.
Once you have been to all of these biomes, which can be found fairly easily by simply exploring the Nether, the achievement should unlock.