Anything that can be seen, heard, smelled or touched can be a symbol, but it can also be an action like a dance or have rules to follow like a ritual. All it needs is to have a strong belief or feeling attached to it. An example of this is the act of putting salt in a window to block evil spirits from entering. Many could say that this is just an old wives tale but the influence of symbols should not be ignored. The power of symbolism is already quite present in our daily lives with all the religions, traditions and even publicity.
A symbol has the power to inspire and control. Like a stop sign or a flag. It is believed by many that the influences go even further than that. The belief is that the more you believe something the more you give it power. That could enable it to transcend the limitations of reality to some extent. Now imagine millions of people having the same belief about the same something as you. This influence is completely invisible and hard to prove or grasp, but in our path to grow, resolving this issue is essential for our own personal evolution.
A symbol is a connection to the invisible, giving it more value and influence. A material amplifier.