This is where yeast is added and fermentation takes place and alcohol is produced for the first time.
"The cooled wort is then transferred to the Oregon Pine Washbacks. Edradour has two Washbacks, both over 50 years old."
http://www.edradour.com/~edr99/explore/unique-features/
Edradour's claim to fame is that they are the smallest distillery in Scotland.
https://en.wikipedia.org/wiki/Edradour_distillery