What does Zechariah 4:7 mean?

Who are you, big mountain? In front of Zerubbabel, you'll become flat ground. He'll bring out the capstone with people shouting, 'God bless it! God bless it!' Zechariah 4:7 - Modern Text Bible

(Obstacles that seem huge will be leveled before Zerubbabel. The capstone is the finishing stone of the temple, and people will celebrate God's work.)