A working candle is a candle that one uses as they are doing a magical working. An example would be a candle that sits on my alter and I light before I start working on any magical working such as anointing a candle, construction a charm or other magical object, meditating, writing a spell, etc.
Some will use the candles that are lit for deity while others will have a separate candle just for working. A working candle will be the one that gives the representation of the element of fire as well as the candle one might use wax from for the construction of an item. Such as when you need to seal with wax or adhere one item to another with wax. It also includes times when you need to light a match or stick with existing flame, use a flame for consecration. etc.