A brelfie is a breastfeeding selfie. That is the whole definition. It does not have to be glamorous, public, or perfectly lit. If you captured a feeding moment, you made a brelfie.
The word caught on the same way selfie did: people needed a shorthand for a picture they were already taking. Some brelfies stay in a camera roll. Some go on Instagram with a caption about supply, solidarity, or a quiet Tuesday. Both count.