CPD stands for Continuing Professional Development. It refers to the process of tracking and documenting the skills, knowledge and experience that you gain both formally and informally as you work, beyond any initial training.
CPD is a record of what you experience, learn and then apply. The term is generally used to mean a physical folder or portfolio documenting your development as a professional. Some organisations use it to mean a training or development plan but this is your organisational decision to make. Its function is to help you record, review and reflect on what you learn.