"Throng" means to crowd around somebody. Here, the Lamb of God is the object.
"Effulgence" is brightness or a brilliant light radiating from something.
"Sorrow now nor thraldom knows" - Thraldom speaks of domination: a condition of being controlled by a more powerful person or force. Unlike the old creation, the new creation does not know the effects of sin, such as toil, sorrow, and bondage.
Hark! ten thousand voices crying
"Lamb of God" with one accord;
Thousand thousand saints replying,
Wake at once the echoing chord.
"Praise the Lamb", the chorus waking,
All in heaven together throng;1
Loud and far each tongue partaking
Rolls around the endless song.
Grateful incense this, ascending
Ever to the Father's throne;
Every knee to Jesus bending,
All the mind in heaven is one.
All the Father's counsels claiming
Equal honours to the Son;
All the Son's effulgence2 beaming
Makes the Father's glory known.
By the Spirit all-pervading,
Hosts unnumbered round the Lamb,
Crowned with light and joy unfading,
Hail Him as the great "I AM".
Joyful now the new creation
Rests in undisturbed repose,
Blest in Jesus' full salvation,
Sorrow now nor thraldom knows.3
Hark! the heavenly notes resounding,
Higher swells the song of praise;
Through creation's vault responding
Loud Amens which joy doth raise.