Mindfulness* is knowing.
However there are no words or concepts that can accurately describe what mindfulness is. Mindfulness is something you experience. You can not know mindfulness by studying ideas (theories and concepts) or imagining because mindfulness is not an idea. A person needs to experience it in order to know it. Come and experience for yourself.
* sati in Pali, 念 in Chinese, 알아차림/마음챙김 in Korean.