In
Divergent, the "test" is a key part of the initiation process for the factions, which are groups in society based on certain virtues like bravery, intelligence, honesty, selflessness, and peace. The test helps determine which faction a person is best suited for based on their characteristics and responses to different situations.
For the protagonist, Beatrice (Tris), the test is supposed to show which one of these factions is the best fit for her. However, her results are unusual because she shows traits from multiple factions, which is considered dangerous. This is what makes her "Divergent."
The main purpose of the test is to evaluate where people belong, but it also serves a broader purpose in the story, as it reveals how the system is controlling and suppressing individuality. Those who don't fit neatly into one category (like Divergents) are seen as threats to the control and stability of the society. So, while the test is meant to assign people to factions, it also plays a role in the larger theme of conformity versus individuality in the story.