what to do

[kona]

the wee bottle like bit of my exhaust(just forward of the back wheel) decided to detach itself.
i have searched through the site here and would guess im looking at getting a mongoose replacement. Im mechanically ignorant and have not a clue what a DECAT is,it seems important.i need the exhaust to be able to pass the NCT.
any advice would be a great help.
its for a GR