"For He put on righteousness as a breastplate, and a helmet of salvation on His head; He put on the garments of vengeance for clothing, and was clad with zeal as a cloak." (Isaiah 59:17)
God Himself is zealous and fervently passionate about His people and His plan. In simplest terms, zeal is fuel. It is anything that drives people: their passion, their purpose, what they live for. For Christians, zeal is a burning desire to do God's will and to live according to His purpose.