The light emitted by the candle symbolizes the light of Christ, the very presence of the Lord in our lives, but also the hope for eternal life, the life as it is called of the inexhaustible light.
As a symbol, the candle functions in the life of the believers as a reductionist, like all other kinds of piety, lighting candles, offering incense
In the Old Testament the god commands Moses to advise his brother as follows
Command Aaron and his sons to light a sleeping lamp from the afternoon until the morning before the Lord with what is prescribed by the law and is eternal in all your generations.
The custom of placing candles in tombs and houses is an act of piety of Christians that pleases God and the Saints and together soothes the souls of the dead.
Regarding the lighting of candles in the graves of the fallen, Saint Nicodemus of Mount Athos says that
Candles are lit for the forgiveness of the sins of those who offer it, as long as they are lit up to God and to the Saints, so are the sins of those who are either alive or dead, as Martyr Demetrios said to the apostle
And Agios Kosmas the Aetolian advised
My brothers, don't grieve for your dead, but if you love your dead, do what you can for his soul: quarantines, memorial services, liturgies, candles, frankincense, oil, fasting, prayers, almsgiving. May God show his mercy and save your deceased and take him to Heaven or is his hell lighter. And as many women as you wear soiled by your deceased should take them off, because you are harming your word, you are also harming your deceased.