It's symbolic content can be understood in terms of two: two covenants and two flocks becoming one in Christ, the Good Shepherd. Twelve is an obviously significant number since there are 12 tribes in Israel and twelve apostles. 24 is merely the whole of twelve times two, and so represents the one people redeemed by Christ out of Israel and the Gentiles. That is clearly a major theme in the Apocalypse, though it leads dispensationalists and non-dispensationalists to vastly different conclusions.
This theme is visited rather dramatically for the last time in chapter 21 as the New Jerusalem which comes down out of heaven to a new earth is described. The eternal home of the saints has twelve foundations and twelve gates. The combination of 12 and 12 in the structure of the New Jerusalem (which is 24, though not explicitly mentioned) is used to encompass the entirety of God's salvivic people, and picks up the theme which streams throughout the Apocalypse. Jew and Gentile who believe in Christ, though distinctive in some ways, form one eternal people of God.
The 24 are elders (presbyters) which means, basically, they are old men who are wise and worthy of respect. I think the use of the generic term, "elders," accentuates their symbolic quality, and yet excludes seeing them as non-human living creatures, or even angels, because those things are specified in the Apocalypse when they are meant. How long they've been there, or how they got there is not mentioned, so it's either unimportant or so obvious it's assumed to be known. Could they represent the sons of Jacob and the twelve apostles?
Although John is viewing and recording the vision, not much of an objection could be raised to the 24 representing the 12 Apostles (Paul substituted for Judas). It's a bit more difficult to see them representing the actual, less than exemplary, sons of Jacob. Throughout biblical history the names of the twelve tribes was always more important than the twelve people that gave those tribes their names, so specification as to person is not so important with the twelve representing Israel, which fits well given their generic identification. I suppose they could represent some exemplary member of each of the associated tribes, but it's not necessary.
They are given thrones placed in close conjunction with that of God, which, along with their victory (but not regnal) crowns, implies they are engaged in judgment and administration with him. That jives well with Matthew 19:28, which would tend to verify seeing at least twelve of them as representing Christ's Apostles. If that is the case, then it's hard to avoid the math and see the other twelve as faithful representatives from each of the twelve tribes. They are clothed in white which is always associated with purity or righteousness in the Apocalypse, so, in effect, the 24 elders are clothed in righteousness.
Aside from judgment, the 24 seem occupied with worship. They hold censers and harps. They fall to their knees (the implication of proskuneo), cast their victory crowns at the feet of God, extol the Creator's virtues, and sings songs of praise to God and the Lamb. The force of their worship is to attribute to God the action that accomplishes his salvivic and magisterial aims--God is the actor, everyone else is the benefactor.
We are told explicitly that the incense signifies the prayers of saints. That is not an endorsement for the doctrine of the Intercession of the Saints, but merely represents that the prayers of the saints rise directly before God. The elders, though themselves men and therefore representative in some fashion of all believing humans, are not the makers nor mediators of the prayers (interceders), but, really, only witnesses of such. The harps, in very similar fashion, signify the praise of those same saints.
So the prayer and praise of the saints rises to the throne of God, symbolically carried by those representative of all who follow. As they are before God in prayer and praise, symbols in the heavens, so are those they represent also before God as they praise and pray on earth.