It means “cold.” In the past, the R was rolled (still true, for example, in Scotland), sounding a lot like shivering or teeth chattering from the cold. By extension, “cold” could mean unfriendly or unpleasant in general.

"Brr" is an expression used by English speakers to indicate that it is cold or they are cold. I live in an area of Canada wherein winters can be very cold. To emphasize the cold temperatures a person will sometimes use it as an informal description for how very cold it is; for example "it's brr cold outside." or " brr that's cold." or "I'm brr cold." If we simply say a big "brrrrr", then native speakers here know the meaning.

As well as physical cold, I think it's acceptable to use "brr" to mean "the very idea turns my blood cold". That would signify a dislike of something. "Eww" is a sound of disgust.

Very simple - 'brr' is the sound you make when you're cold!

You can read it as 'Burn' moving off the 'n'.