The mind and the breath are fundamentally linked. In Yoga practice we work with each to influence the other.
When we are anxious, fearful or disturbed in some way, the breath can become shorter, faster and irregular. Likewise, when we feel content, the breath has qualities of being longer, slower and even. Prāṇāyāma is a practice that supports change in this respect.
Āsana practice incorporating precise use of the breath with movement is an important preparation towards integration of the breath in prāṇāyāma.