The great commandment is what Jesus said when someone asked him which is the greatest commandment of God. He said that the first and most important one is to love God with all your heart, soul, and mind. That means you should really care about God and try to do what He wants you to do. You should listen to Him and follow His teachings.
The second most important commandment is to love your neighbor as yourself. That means you should care about other people just as much as you care about yourself. You should be kind and helpful to them, and you shouldn't hurt them or do anything that might make them feel bad.
So, the great commandment is all about loving God and loving others. If you do these things, you will be following God's greatest commandments, and that will make Him happy.