Both the madcap Hungarian Diak and Brother Kemner are right on track.
The Greek: sym-boulon (throwing together) is probably better represented in root English by the Latin form: com/con-pilatio, or "compilation". That is: various elements that constitute a reality are brought together to create a new entity. The "compilation" has its own essence, that is, it has existence in itself as an entity, but there is a correlation between the "compilation" entity and the spiritual reality that it is meant to represent. Thus, the "symvol vieri" (the statement of the Creed) is a reality in itself, but it has a correlation with the spiritual realities which the Creed professes to exist.
In some ways, I see it as an analogue to the way that we interpret icons. The icon "re-presents" (presents-again) what the icon depicts. The icon has its reality (hey, we can physically weigh it, measure it, see it, etc.) but so does the reality that the icon represents. Thus, we are dealing with a type of parallelism: reality (spiritual realm) = reality (physical realm).
As a result, I believe that we humans have a tendency to understand the physical re-presentation as a critical phenomenon that ties us to the spiritual realm. Thus, we venerate icons, we bathe the Gospel book in golden covers, and we consecrate the church building through anointings; why, because these physical realities provide us with an open door to the saints, the Received Word, and the sacraments (seriatim).
The physical representation is more than a "compilation", as the language would seduce us into believing, but rather something that has reality (no question!) but also a true spiritual/non-physical linkage to a spiritual reality.
The critical element is the ability of us human beings to inextricably and "essentially" link the physical reality to the spiritual reality. Thus: sacraments and sacramentals. To refer to the old Baltimore Catechism (an oldie, but a goodie!): A sacrament is an outward sign instituted by Christ to give grace.
So, the "symvoles" are outward signs, instituted by Christ (we propose) to give grace.
[Psychics, horoscopists, tarot-ists, etc. use this same logic to 'affirm' the reality of their predilictions. Links between spiritual realities and physical/human reality. Symvoles.]
The critical difference is that our Christian affirmations are connected to the ethical responsibilites of the faith, and have an essential connection to our lives as we live them and are not just willy-nilly "realities" presented by the cards or the entrails or the stars that are supposed to have an imprint on our living realities.
The whole notion of "symvolos" is a fascinating journey through our human understanding of how the day-to-day lives of people are related to the realities of the non-physical/spiritual world. As Jews, Christians and Moslems, we have an understanding of how the God of Abraham, Isaac and Jacob thrusts Himself into our human affairs. (It is RADICALLY different from Hindu, Buddhist, Confusian, Euro-pagan, animist, etc. understandings of God's relationship to people and His creation. And while there are certainly parallels that are incredibly valid and valuable, we must never inter-change these perspectives.)
Thus, our "symvolos" concept postulates a direct linkage between the human reality and the spiritual reality, exemplefied in the symvolos itself. We respect the symvolos (in our world-view) because it is a corresponding reality to the spiritual entity. But we must be ever vigilant to ensure that what we propose as "symvolos" for a spiritual reality is in absolute, complete and incontrovertable accord with what the scriptures and tradition teach. Otherwise we are just doing the soothsayer thing, approaching Madame Zelda for a reading, and hoping that her "visions" and "readings" are a true representation of reality. [Sure. And for twenty bucks I'll give you a never-failing lottery number. Unmarked bills please.]
Hope this works for your needs.