Mindfulness is about learning to pay attention to the present moment in an accepting and non-judgmental way. Often, our mind can take us into negative thought spirals before we realise that we are no longer aware (or mindful) of our surroundings or thoughts. Mindfulness can be helpful in learning to observe and become more aware of your experiences, as well as increasing your attentional control.
Although relaxation can sometimes be a pleasant side effect of practising mindfulness, this is not the main aim. It is normal to find it difficult to practise mindfulness, or even become frustrated because your mind drifts to other thoughts. However, it is the very act of noticing that your mind has wandered and bringing it back to the exercise or task that is an important part of increasing the strength of your attentional control.