God is good. He provides for our every need. In this fourth petition, we ask that God give us not only the material needs which sustain our physical lives, but more importantly, our spiritual needs which keep us on the path of everlasting life and peace. This latter need is what the world is truly starving for.
The following link will take you to a simple explanation of this petition written by St. Pius X (a pope in the early 20th century): The Lord's Prayer--the fourth petition