As the name suggests, character development relates back to (wait for it) the development of the characters.
What does that actually mean? Characters are supposed to read like real people, which means that they need to have depth.
How do you go about discovering that depth? Throw your characters into situations and see how they react. Push them into wild new challenges and watch how their personality comes out, look for the little snippets of flaws, history, and more. That’s your development.
Flat characters which read like cardboard cutouts are tedious and very likely to bore the reader. Let your characters loose and embrace them as (imaginary) real people.