Concise Bible Dictionary:

στοιχεῖον, “rudiments, first steps.”
1. Applied to children at the “commencement” of their training; and to the law as the “early” way of God’s dealing with Israel; but now called “beggarly” because it has lost its glory through the failure of man, and the introduction of Christ Himself (Gal. 4:3,9). The word, with a similar meaning, is translated “rudiments” in Colossians 2:8,20, and “principles” in Hebrews 5:12.
2. The material elements of the universe, which will be melted with great heat in the day of the Lord (2 Peter 3:10,12).