It represents the three significant ages or stages of faith in God.
The first is (Jod) the age Jehovah the father - who is seen as the father of all creation. The second is (He) the age of the son who bears the image of God - who is seen as the sacrificial lamb who takes away the sins of all the world. The third is (Shin) the age or the Holy Spirit - who is seen as the one that gives total liberation to the believers.
These three ages represents the three manifestation of God on earth as the Father, Son and Holy Spirit.
It is this mystery that is represented in the altar of a church called Celestial Church of Christ.