What does it do?

I tried it on Lubuntu 16.04; it required modification:

   # mystery one-liner from https://news.ycombinator.com/item?id=13513171
   # dpkg -l 'linux-' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]\)./\1/;/[0-9]/!d' | xargs -p sudo apt-get -y purge
   # my corrected/improved version:
   noncurrent_kernel_pkgs() { dpkg -l 'linux-*[0-9]*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.\)-\([^0-9]\+\)/\1/")"'/d;s/^ii *\([^ ][^ ]*\)[^ ]*.*/\1/' ; }
   # corrections/(possible-)improvements:
   # - dpkg -l 'linux-' # returns nothing
   # - dpkg -l 'linux-*[0-9]*' # moves 'pkgnm must contain a number' rule (last sed cmd in orig: '/[0-9]/!d') forward
   # - pkgnm isolation/extraction was broken (did dpkg output change?)

output:

  kg@kg-VirtualBox:~$ noncurrent_kernel_pkgs
  linux-headers-4.4.0-43
  linux-headers-4.4.0-43-generic
  linux-headers-4.4.0-45
  linux-headers-4.4.0-45-generic
  linux-headers-4.4.0-53
  linux-headers-4.4.0-53-generic
  linux-headers-4.4.0-57
  linux-headers-4.4.0-57-generic
  linux-image-4.4.0-43-generic
  linux-image-4.4.0-45-generic
  linux-image-4.4.0-53-generic
  linux-image-4.4.0-57-generic
  linux-image-extra-4.4.0-43-generic
  linux-image-extra-4.4.0-45-generic
  linux-image-extra-4.4.0-53-generic
  linux-image-extra-4.4.0-57-generic
  kg@kg-VirtualBox:~$ uname -r
  4.4.0-59-generic
  kg@kg-VirtualBox:~$

Sorry about that, the '*' got removed from a non marked up paste. You are 100% correct. Just a little script to remove all but the current unused kernels that I use on 14.04. Yep, just take the xargs off to see what it will remove (which would always be good advice when pasting a random script LOL ).

Take off the xargs part of the command and run it to see. :-D

Just looking at it though, it looks like it'll purge kernel packages that are the currently booted one. I don't have a system handy to check though so take that with a grain of salt.

Just for posterity that should have been "are not the currently booted one".