Mindfulness is an awareness of the present moment. This awareness is one in which people are trained to take the stance of an observer towards their thoughts, feelings, and experiences. It is different than getting caught up in the moment, in the feeling or sensation or getting preoccupied with thoughts about what it all means. This can be an incredibly useful skill, but is not an easy
Noticing how one feels in their body, what emotions they are feeling and thinking can be especially difficult if they have any kind of complex trauma or dissociation. This is because it can be very challenging and scary to notice what is going on inside. However, if it is tolerable, then it can be really helpful because this observational mode, then we are not caught up in judging or evaluating the present moment, we are not going over our past failings, not worrying about what might happen in the future. And one of the best things it helps you learn that thoughts, feelings and sensations come and go. That is one reason that people struggle so much is because they feel like the unpleasant moment is going to last forever, but nothing lasts forever and this is one way to learn that.
Form more resources and information on mindfulness, look into the teachings of Jon Kabat-Zinn, PhD. He is a clinician who popularized the use of mindfulness as a therapeutic technique as part of his Mindfulness Stress Reduction and he is a world renowned teacher, researcher, author and clinician.