Mindfulness comes out of the tradition of Buddhist meditation practice. In mindfulness meditation we develop skills of attention, learning to monitor, note, and release cognitive reactions ‑‑ thoughts/judgments ‑‑ about our perceptions, as they happen.
Skills of attention are central in the CBT work as well, though in therapy, rather than noting and releasing all thoughts that rise, we seek out the troublemakers, the irrational ones that may be driving our dysfunctional emotions and/or behaviors. Once found, we actively question their purpose and dispute their validity.
I see CBT as a form of applied mindfulness and often recommend that clients learn mindfulness meditation as an adjunct to treatment.