Meditation is above all a quest. The mind seeks to understand the why and how of the Christian life, in order to adhere and respond to what the Lord is asking. The required attentiveness is difficult to sustain. We are usually helped by books, and Christians do not want for them: the Sacred Scriptures, particularly the Gospels, holy icons, liturgical texts of the day or season, writings of the spiritual fathers, works of spirituality, the great book of creation, and that of history the page on which the "today" of God is written.
Source: Catechism of the Catholic Church (Vatican.va)