It is known as the Jerusalem Cross because all it represents stems from Jerusalem.
The large cross in the center represents Jesus, Our Saviour who lived, died on the cross and rose again in Jerusalem in 33 A.D. At that time Jerusalem was the center of the world and the center of Christianity.
The four small crosses are symbolic of the four Gospels; Matthew, Mark, Luke and John as well as the four corners of the earth which Jesus commands His disciples to spread the Gospel to;
And Jesus came and spoke to them saying “All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the son and the Holy Spirit, teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age.” Matthew 24:18-20.
It is a beautiful and unique design symbolizing the spread of Christianity across all denominations of Christians, starting from Jerusalem and spreading to the four corners of the earth.