# fell down
"lay down on the ground." Their faces were toward the ground to show that they were worshiping the Lamb. They did this on purpose; they did not fall accidentally.
# Each of them
Possible meanings are 1) "each of the elders and living creatures" or 2) "each of the elders."
# a golden bowl full of incense, which are the prayers of God's holy people
The incense here is a symbol for the believers' prayers to God.