what protestants don't understand about what catholics believe about Mary

Mary, Did You Know? | Desiring God 

catholics don't believe that Mary is God. 
catholics believe that there is one God.
God has 3 persons: Father, Jesus and Holy Spirit.
Mary is not God.

catholics believe that, other than Jesus, Who became man, died and rose from the dead, Mary is the most obedient-to-God human being who ever lived.