No one knows. It is said that 24 is a multiple of the sacred number 12 (a number of completion of the people of God), and so it is guessed that 24 is all-inclusive of salvation history.
In ancient times, elders were representatives of the wisdom of their community, and therefore judges.
The theories on who these elders are:
1. Glorified human beings. 24 human beings representing all of the redeemed. In Acts (15.6; 20.28) the elders are officials in the church who represent the whole church. In the OT, 24 elders were appointed by David to represent the entire Levitical priesthood (1 Chron. 24). (Ryrie p. 35-36; Vincent p. 478; Feuillet) (Kittel [Vol. 6 p. 668] argues that there is no suggestion that they are redeemed and transfigured [see vv. 7 & 14] men.)
2. Angels (Kittel [Vol. 6 p. 668]; Allo) argues that there is no suggestion that they are angels, for they are distinguished from the angelic hosts in 5.11 & 7.11.)
Though angels are never said to occupy thrones, crowns are never ascribed to angels, angels are never elsewhere called elders, and in 19.4, the elders are said to have been redeemed.
3. Israel (Ford p. 80). They are associated with the kingship or the ruling power over the State (Sirach 44-49), in contrast to the kings of the Earth.
4. A distinct class of heavenly intelligences
5. Princes of heaven; “Tribal heads”
6. A council of heavenly beings (Newell)
7. Prophets