Thief Evasion feature allows two things:
- Against effect area spells that trigger dexterity save throws, if the thief fails the save roll, he is dealt half damage (instead of taking complete damage).
- Against these same spells, if the Thief succeeds the saving throw, he does not suffer any damage (instead of half damage)
You may be thinking about Uncanny Dodge functionality that Thieves can use to halve damage by entering, using their reaction, each time they are attacked.
These are two different characteristics that compensate for the lack of survivability of the thief, but have different mechanisms and interactions: one (Evasion, at level 7) for spells, and one (Uncanny Dodge, at level 5) for regular attacks.
the Uncanny Dodge functionality should not be confused with the To dodge Action: The action that Thieves (or any other character) can perform during their turn instead of taking any other action that confers a disadvantage on enemy attacks against them.