Lighting a Unity Candle during your wedding ceremony is a special way to symbolize your two lives joining together as one.
Generally there will be one central candle symbolising your new family, with two smaller candles either side symbolising your current family. Sometimes these candles are decorated and labelled prettily. There can be taper candles as well.
Often (but not always), the mum will light the individual family candles at the start of the ceremony, and then once you are husband and wife, you will light the single candle from your family candle, and snuff out your old family candle, symbolising the start of a new family – that of you and your spouse.
This ceremony can add a religious feel to your ceremony – if you want it to be seen that way. And obviously it is more suited to indoors than outdoors; although there are ways that it can be done outdoors to lessen the impact of wind.