There is much wisdom in this passage from a recent article by Fr John Breck:

As it is used in patristic tradition, "watchfulness" implies an inner attentiveness or vigilance. It requires wariness in the face of attacks from both within and without, from our worst inner impulses and from the onslaught of demonic temptations. Accordingly, watchfulness is a key element in spiritual warfare.

The eighth century ascetic writer Hesychios of Sinai composed a remarkable treatise on "watchfulness and holiness," included in the Philokalia. He begins with this description: "Watchfulness is a spiritual method which, if sedulously practiced over a long period, completely frees us with God's help from impassioned thoughts, impassioned words and evil actions. It leads, in so far as this is possible, to a sure knowledge of the inapprehensible God, and helps us to penetrate the divine and hidden mysteries. It enables us to fulfill every divine commandment in the Old and New Testaments and bestows upon us every blessing of the age to come. It is, in the true sense, purity of heart..."

Read it all here.