Being aware of your breath forces you into the present moment — the key to all inner transformation. Whenever you are conscious of the breath, you are absolutely present. You may also notice that you cannot think and be aware of your breathing [at the same time]. Conscious breathing stops your mind.
Eckhart Tolle

Mindfulness of breathing meditation is merely focusing attention on: breathing in … breathing out.

ana … pana … sati
/ | \
breathe in … breathe out … be here now

The center of our practice is the breath.

We don’t try to control the rate, length, or depth of breathing in any way. We just maintain mindful awareness on the natural rhythm of breath as it happens in the present moment.

To help focus the attention, the meditator can make a mental note “in” while breathing in and “out” while breathing out.

Breathing in, i see myself as clear water.
Breathing out, i accept things as they are.
Thich Nhat Hanh

The Buddha gave several teachings on the practice of anapanasati. The two main ones are the anapanasai sutta and the satipatthana sutta.

More about meditation.