Zechariah 6:7
And the bay went forth, and sought to go that they might walk to and fro through the earth: and he said, Get you hence, walk to and fro through the earth. So they walked to and fro through the earth. And the bay went forth, and sought to go that they might walk to and fro through the earth: and he said, Get you hence, walk to and fro through the earth. So they walked to and fro through the earth.
The bay horses belong to a series of symbolic visions given when the Jews were anxious about whether they could continue rebuilding. That series was meant to sustain and encourage God's people by revealing His compassion and care.
The bay horses are heavenly agencies sent out from before the Lord. They go throughout the earth under His command to walk to and fro through it. Their going shows that the Lord's eyes are open to the work of His cause everywhere.