What the two candles signify
Two candles precede the Gospel because the law and the prophets, which preached the Gospel, preceded it. The two candlesticks that are carried before it are the two precepts of charity taught by the Gospel. The two acolytes who carry it are Moses and Elias, between whom the Lord shone like the sun on the mountain (Matthew xvii, Mark ix, Luke iv). While the Gospel is read, the candles are placed on the floor because the shadows of the law and the mysteries of the prophets are understood by the humble through the light of the Gospel. After the Gospel is read, the candles are extinguished, for it is understood that they were spiritually brought forth by the light of the Gospel. The thurible is carried before the Gospel, for Christ who is sacrificed for us in the fire of his Passion is preached as an odor of sweetness.
 Jungmann offers a commentary on the Gospel candles: “The carrying of tapers before the Gospel tallies with an ancient Christian practice that must have been common to all the liturgies. In fact St. Jerome testifies that it was customary in all the churches of the Orient to light lights when the Gospel was to be read, and this on the brightest day; in this way an air of joy could be lent to the gathering [….] We can also recall the custom of carrying lights and incense before the bishop at a solemn entry–one of the honors which, since the time of Constantine, was transferred from the higher civil officials to church dignitaries” (Missarum Sollemnia, 445-446).
Quid duae candelae designant.
Duae candelae Evangelium praecedunt; quia lex et prophetae praecesserunt, quae Evangelium praedixerunt; duo candelabra quae praeferuntur, sunt duo praecepta charitatis, quae per Evangelium instruuntur. Duo acolythi qui ea portant, sunt Moyses et Elias, inter quos Dominus ut sol in monte fulgebat. Dum Evangelium legitur, cerei in pavimentum deponuntur, quia umbrae legis, et aenigmata prophetarum per lumen Evangelii ab humilibus intelliguntur. Perlecto Evangelio candelae exstinguuntur, dum per lucem Evangelii spiritualiter prolatae intelliguntur. Thuribulum ante Evangelium portatur, quia Christus odor suavitatis in igne passionis pro nobis sacrificatus praedicatur.