One of the joys of the Catholic faith is that we have outward signs and symbols to help us express and deepen our faith (such as in the Sacraments and sacramentals). Candles can be seen as outward signs of our offerings to God and requests made to the saints. Some people may like to offer up a prayer before, during or after lighting a candle. Candles can represent someone’s intentions, e.g. for the repose of the soul of a friend/family member, for assistance in illness, etc.
Sometimes though we may be lost for words - I know there have been times when I have been so full of gratitude, or feeling so lost/upset, that I did not have any words I could use and so, for me, lighting a candle was an outward sign of offering up all that was in my heart (think as well of how a flame points upwards reminding us of who and where our petitions are addressed to).
There’s also the symbolism of light - Jesus said,‘I am the light of the world’ and we are called to shine as lights in the world for him. Wherever light is it banishes darkness and allows us to see more clearly - prayer does the same.
Does that help at all? Sorry if it seems a bit disjointed, I just wanted to give a quick answer, however, there is so much more that could be said and some may have a different approach - you can find some great little books and leaflets on Catholic practices in most good religious bookstores and there are some good articles online if you’d like to explore things further.
