There are 4 biomes, each representing a chapter of the story. You start in Spring, then move to summer, then autumn and finally winter. You cannot return to previous biomes, and you can choose when you want to move forward by performing the ritual at the center of your village. For that reason, make sure you've finished whatever you need to before performing the ritual.
With regards to this achievement, you must simply travel to each island within a biome for the a judgement to pop. You will get this achievement in the process, if you are also going for the achievement to clear every island of its corruption within a biome.
While simply walking on the island for a few seconds should be enough to register as having visited, I would recommend getting the pop up that tells you the name of the island to be safe. With the knowledge that islands only have names once you have visited them, you can check which islands you have visited within your map screen. Islands with '???' are safe to assume that you have not yet been there.
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