What is the purpose of the stuff (outlined in RED) and what is made out of?

 

Its sound deadening material. The trunk pan can resonate and boom like a drum or a speaker if you leave it un-damped.

Its similar to dynamat, but stiffer. Its made of like a rubberized tar/glue. Maybe some ceramic substrate in there.

 

You will find something like it on the transmission tunnel under the carpet and under your metal kitchen sink.