When you want to extinguish a flame, be it of a candle, or a match, or whatever, do not blow it out with your breath. You can shake it out, or wave your open hand quickly close by it, or if in a safe place, throw it to the ground.
The word for breath in Hebrew is Neshama. Neshama is also the name of one of the levels or dimensions of our soul. Wind is Ruach. Ruach also means spirit and is the name of another level of our soul.
Is this a negative commandment from the Torah; thou shalt not extinguish a flame with your breath? No, blowing out a candle is not forbidden in the Torah.
Is this superstition? Do we think that blowing out a candle with our breath will lessen our soul, or lessen the spiritual light in the world? No. But it is a Jewish custom for a good reason.
We are told not to use our breath to extinguish light to remind us to use our breath to share light, to help to make the world a brighter place. Our soul, our spirit, our breath… these come into the world to spread light, not to extinguish it. In fact, we are told to be a "light unto the Nations".[i]
[i] Isaiah 49:6