Star levels change the appearance of your cards by adding special effects around them.
The first star upgrade adds glowing borders and makes your cards look shiny in battle.
They do not affect gameplay balance or any card stats like damage or health.
Star levels are purely cosmetic and are earned by collecting duplicates of Legendary cards.
It is a fun way to show off skill and progression but do not expect improved performance.
They give collectors or competitive players a prestige factor without gameplay changes.