Codependency is a well worn "buzz word" meaning when one person takes on some of the unhealthy characteristics of the unhealthy person they are in relationship with. One may have a sense of over-responsibility and the other person's problems become your problem. Self worth becomes dependent on the actions and attitudes of others and based on the approval of others.
There's often a sense of needing to solve the other persons problems, to give more in order to be loved. Symptoms of codependency may include: an intense fear of rejection, the need to control or the perception of being controlled.
Boundaries are designed to, define one self in, and live out healthier relationship. It is difficult to commit to what you will accept and what you will not accept when in a codependent relationship. You may waver on what you will do and not do, say and not say. Responsibility is one of the most freeing concepts in relationship and also can be one of the most distorted and confusing aspects between two people. Responsibility is a reciprocating aspect of boundaries and among other concepts allows for a healthy interdependence between two people (not independence nor dependence).
Codependency can stem from early experiences learned in relationship. Sometimes this learning is ingrained and requires some deeper healing work. It is possible to move past the pain, heal and design life around healthy boundaries.