Duplicated string literals make the process of refactoring error-prone, since you must be sure to update all occurrences.
On the other hand, constants can be referenced from many places, but only need to be updated in a single place.
Noncompliant Code Example
With the default threshold of 3:
action CONSTANT VARCHAR2(7) := 'action1';
To prevent generating some false-positives, literals having less than 5 characters are excluded.