They can also be used as tools. As an apparatus to help you focus on a given issue, as part of a meditation or prayer ritual, or, many people believe, as powerful additions to a spell or charm to help you achieve your desires. Candles play a large part in many Catholic rituals. Many Wiccans believe that they can add power to a spell or to their "magic" by using a candle.
It is your words, your hopes and your intent that make any ritual have meaning. Any tool, such as a candle or incense, is used to help you focus your intent and make you "one" with your ritual as you request Divine assistance. Whoever you understand the "Divine" to be. Many people believe that we create the reality in our lives through our thoughts. Candles can help you get focused on just exactly what those thoughts are.
Some people believe that "dressing" or anointing the candles with oils and/or loading the candles with herbs will also lend more specific energy to their ritual. There are many good books available on the magical use of candles with oils and herbs. It is also possible to inscribe the candle with a sigil or sigils to help you achieve your intent.
Candles represent the the power of light out of darkness. Candles represent knowledge and spiritual spiritual llumination. Ancient civilizations used fire to symbolize the life force within each of us.
They can also be used to represent the Elements in ritual or in Feng Shui. Certain colored candles are said to have particular meanings when used as tools as described above.