Mindfulness is the idea of learning how to be fully present and engaged in the moment, aware of your thoughts and feelings without distraction or judgment. It is the conscious awareness of our present moment. This includes openness and non-judgment about the experience. It is often coupled with other types of therapy, such as Cognitive-based Therapy (CBT), Dialectical Behavior Therapy (DBT), or Acceptance and Commitment Therapy (ACT).
Mindfulness therapy is not concerned with relaxation, though that might be a result of certain practices. The focus is on increasing our awareness of the thoughts, feelings, and actions that hinder our progress. When we are better able to do that, we can engage with those aspects of ourselves, learn to tweak our language, and choose how to respond.