What ##*/ does in bash?

Question

From the script:

https://gist.github.com/jonnyparris/0a10cc63af281de23a4fd34116fed3e6

I can't figure out what the second line of the snippet below does. I assume it should be a kind of trim.

BRANCH_NAME=$(git symbolic-ref --short HEAD | grep -o '[A-Z]\+-[0-9]\+')
BRANCH_NAME="${BRANCH_NAME##*/}"

Any reference or explanation would be really appreciated.