Meditation can be instrumental in spiritual growth and can be used as a tool to improve the effectiveness of prayer. Meditation differs from prayer in that there is a distinct effort to fully absorb small doses of God's Word in order to accept the healing and strength that it offers.
In eastern meditation the object is to empty your mind, while in Christian meditation the object is to fill your mind with Christ and God's Word. If you are unsure if an app or other tool to learn meditation is safe for you to practice, check with a trusted pastor.
A great way to learn to meditate is to read the chapter on meditation by Richard Foster in Celebration of Discipline: The Path to Spiritual Growth. In the book he provides great examples that will help you learn how to meditate. Another great way to start is with the Five Minute Christian Meditation. It provides a simple way to meditate with no training.
Having an app for meditation on your smartphone gives you the tools you need to meditate whenever you want or need to meditate. Meditation can used very effectively as a short break during the day in order to reinvigorate to meet the remaining challenges of the day. Also, many smartphone users keep their iPhones on their nightstand, and meditation is an excellent way to wind down for a night of peaceful sleep.