Candles represent Fire (witch represents the God), and are often the most visible element on an altar. When you light a flame you are repeating the Goddess’ act of First Creation, the candle flame bursts forth from darkness, even as the God burst forth from the Goddess in an explosion of flame and light.

The flame generates energy. Each candle that you light puts out energy of its own, which will add to your working; it acts as a kind of “battery” raising the available energy level, and increasing your ability.

Candles on the Alter
The number and color of candles you want on your altar is entirely up to you. Beeswax candles are considered preferable, but in practice most people buy regular candles at the local store, and they work perfectly well.
Some people use only a single candle on their altar, to represent Spirit, or Primordial Deity.
Some people use a matched or contrasting pair to represent Goddess and God. In such an arrangement the Goddess candle goes to the left, the God candle to the right, in keeping with the Geomancy of the altar.
One might want to use altar candles to symbolize the Four Elements in elemental colors, or the three phases of the Goddess, or whatever conveys the idea o of holiness to you, and helps you to make the magical shift in consciousness. Some may want to include a special candle to honor your ANCESTORS (whether by blood or affinity) or your Spirit Guides.