en.wikipedia.org/wiki/Sanctuary_lamp
Exodus 27:20-21
"You shall order the Israelites to bring you clear oil of crushed olives, to be used for the light, so that you may keep lamps burning regularly.
From evening to morning Aaron and his sons shall maintain them before the LORD in the meeting tent, outside the veil which hangs in front of the commandments. This shall be a perpetual ordinance for the Israelites throughout their generations."
More specifics about the use of candles and why can be found at:
From New Advent (Catholic Encyclopedia)
newadvent.org/cathen/03246a.htm
Briefly:
"…St. Jerome in answer declared that the candles were lighted when the Gospel was read, not indeed to put darkness to flight, but as a sign of joy. (Migne, P.L., XXIII, 345 A.D.)
This remark and the close association of lighted candles with the baptismal ceremony, which took place on Easter Eve and which no doubt occasioned the description of that sacrament as photismos (illumination), shows that the Christian symbolism of blessed candles was already making itself felt at that early date.
This conclusion is further confirmed by the language of the Exultet, still used in our day on Holy Saturday for the blessing of the paschal candle.
It is highly probable that St. Jerome himself composed such a praeconium paschale (see Morin in Revue Bénédictine, Jan., 1891), and in this the idea of the supposed virginity of bees is insisted on, and **the wax is therefore regarded as typifying in a most appropriate way the flesh of Jesus Christ born of a virgin mother.
From this has sprung the further conception that the wick symbolizes more particularly the soul of Jesus Christ and the flame the Divinity which absorbs and dominates both.
Thus the great paschal candle represents Christ, “the true light”, and the smaller candles are typical of each individual Christian who strives to reproduce Christ in his life. This symbolism we may say is still accepted in the Church at large**.
Besides the use at baptism and at funerals (St. Cyprian in 258 was buried praelucentibus cereis), we learn from the so-called Fourth Council of Carthage, really a synod held in Southern Gaul (c.514), that in conferring the minor order of acolyte.
The candidate had delivered to him “a candlestick with a candle”. The usage is observed to the present day.
Such candles as these when carried by acolytes, as we learn from he Gregorian Sacramentary and the “Ordines Romani”, were constantly used in the Roman Ceremonial from the seventh century and probably still earlier. These candles were placed upon the pavement of the sanctuary and not until much later upon the altars. Still the practice of setting candles upon the table of the altar itself seems to be somewhat older than the twelfth century. "
