The rubrics speak to the Sanctus candle being lit from the Sanctus until just after the priests communion during a Low Mass. This candle could be a smaller candle placed upon the altar itself, or it could be a candle in a bracket.
(Msgr. Michael Schmitz, ICRSS, celebrating Mass in Stockholm, Sweden. Photo copyright Daniel Nygård.)
The Catholic Encyclopedia describes it accordingly:
The rubrics of the "Roman Missal" direct that at the Sanctus, even of any private Mass, an additional candle should be lighted and should burn until after the Communion of the priest. This rubric however is much neglected in practice even in Rome itself.
As you might guess from the time the candle is lit until it is extinguished, this practice is symbolically tied to the presence of Christ in the Blessed Sacrament, made present upon the altar by the priest during this part of the sacred liturgy.
It is a beautiful symbol of the sacramental realities, and also a pertinent tie into the character of the priesthood and its relation to not only the Eucharist, but also the Eucharistic Sacrifice.
