i have a question regarding the file descriptors and their linkage in the proc file system. I've observed that if i list the file descriptors of a certain process from proc ls -la /proc/1234/fd i get the following output:

  lr-x------ 1 root   root   64 Sep 13 07:12 0 -> /dev/null
  l-wx------ 1 root   root   64 Sep 13 07:12 1 -> /dev/null
  l-wx------ 1 root   root   64 Sep 13 07:12 2 -> /dev/null
  lr-x------ 1 root   root   64 Sep 13 07:12 3 -> pipe:[2744159739]
  l-wx------ 1 root   root   64 Sep 13 07:12 4 -> pipe:[2744159739]
  lrwx------ 1 root   root   64 Sep 13 07:12 5 -> socket:[2744160313]
  lrwx------ 1 root   root   64 Sep 13 07:12 6 -> /var/lib/log/some.log

I get the meaning of a file descriptor and i understand from my example the file descriptors 0 1 2 and 6, they are tied to physical resources on my computer, and also i guess 5 is connected to some resource on the network(because of the socket), but what i don't understand is the meaning of the numbers in the brackets. Do the point to some property of the resource? Also why are some of the links broken? And lastly as long as I asked a question already :) what is pipe?