Anapana sati, "the meditation on in-and-out breathing, is the first subject of meditation expounded by the Buddha in the Maha-satipatthana Sutta, the Great Discourse on the Foundations of Mindfulness. The Buddha laid special stress on this meditation, for it is the gateway to enlightenment."
In simple words, Anapana sati is mindful breathing. Breathe in and breathe out and be one with it.