We continue in heaven and the worship continues: Whereas chapter 4 reveals God the Creator, chapter 5 is all about Christ the Redeemer. We have a scroll that cannot be opened because "no one is worthy". The scroll was sealed and has so much on it there is writing on both sides (generally scrolls were only written on one side).But then one is revealed who can -- Lion of Judah, Root of David who has conquered (5:5). Lion symbolizes strength and Jesus is a descendent of King David's line (hence Root of David) and through his death and resurrection he has conquered. That is why in (6) he is "a Lamb standing as if it had been slaughtered with seven horns, seven eyes which are the seven spirits of God sent out into the earth." The horn was a symbol of strength (7 horns is perfect strength) seven eyes is one with perfect vision.
When the lamb takes the scroll it becomes occasion for more worship and celebration. There is a song sung to the lamb "You are worthy . . ." Apocalyptic literature often plays with paradox and here and throughout note the play with shepherd and lamb -- the shepherd became a lamb, and the slaughtered lamb has become the shepherd.
Note in verse 11 myriads of myriads and thousands of thousands -- in Hebrew there was no number larger than thousand (no words for million, billion, etc). So anything more than a thousand was thousands of thousands or myriads or sand of the sea shore or stars in the sky (uncountable). The number of angels (11) cannot be counted. Chapter 5 is a prelude -- we celebrate one who can open the scroll and now the seals will be opened.
Remember that the imagery in Revelation is drawn from the Old Testament: see Daniel 7:9-10 for a similar picture of myriads of angels before the ancient of days.