java - Odd syntax in API "String::concat"

I was looking at some of the changes made to the Java SE API with 1.8, and I when looking at the new method Map.merge it shows an example of how to use it with the line

map.merge(key, msg, String::concat)

I understand how to use a lambda expressions to create anonymous functional interfaces, but this seems to use a method as a BiFunction. I like to understand and use obscure java syntaxes, but I can't find any mention of this one anywhere.