Less important or not as important as the main thing.
The word 'secondary' can make people think of things that are not the most important, but still have a place.
Examples:
The secondary character in the story is not as important as the main character.
My favorite color is blue, but I also like the secondary color, green.
The secondary reason I like school is because of my friends.
I like pizza, but my secondary favorite food is ice cream.
In the superhero team, the secondary hero can still do amazing things.
The secondary school is for older kids.