Refactoring means restructuring code to make it clearer, without changing what it does. The safety net is tests: run them before, refactor in small steps, run them after each step. If they still pass, the behaviour didn't change.

Common code smells, signs that code wants refactoring:

Long functions doing several jobs: split them.
Duplicated code: extract a function.
Deep nesting: guard clauses, or extract the inner part.
Long parameter lists: group related values into a dataclass.
Magic numbers and strings: named constants or enums.
Vague names: rename.
Comments explaining confusing code: make the code itself clearer.