A fire was always burning on the Temple altar:
“אֵשׁ תָּמִיד תּוּקַד עַל הַמִּזְבֵּחַ - לֹא תִכְבֶּה.”
“There shall be a constant fire kept burning on the altar; it shall not be extinguished” (Lev. 6:6).
The fire on the Temple altar is a symbol, representing our inner drive for holiness and spirituality.
The kohanim were warned to make sure the altar fire was always burning. We also must guard and cultivate our inner flame:
“The divine thirst, raging and storming with its powerful flame in the heart, must not be extinguished.
A person who extinguishes a burning coal on the physical altar transgresses the prohibition of “There shall be a constant fire... it shall not be extinguished.” All the more so we must never extinguish a lofty spiritual coal on the spiritual altar of the heart, filled with holy life.”
(Orot HaKodesh vol. III, p. 210)