What does the FD column of pipes listed by lsof mean?
Files are not only opened as streams. Some of those are listed in lsof's manual:
FD is the File Descriptor number of the file or: cwd current working directory; Lnn library references (AIX); err FD information error (see NAME column); jld jail directory (FreeBSD); ltx shared library text (code and data); Mxx hex memory-mapped type number xx. m86 DOS Merge mapped file; mem memory-mapped file; mmap memory-mapped device; pd parent directory; rtd root directory; tr kernel trace file (OpenBSD); txt program text (code and data); v86 VP/ix mapped file; FD is followed by one of these characters, describing the mode under which the file is open: r for read access; w for write access; u for read and write access; space if mode unknown and no lock character follows; '-' if mode unknown and lock character follows. The mode character is followed by one of these lock charac- ters, describing the type of lock applied to the file: N for a Solaris NFS lock of unknown type; r for read lock on part of the file; R for a read lock on the entire file; w for a write lock on part of the file; W for a write lock on the entire file; u for a read and write lock of any length; U for a lock of unknown type; x for an SCO OpenServer Xenix lock on part of the file; X for an SCO OpenServer Xenix lock on the entire file; space if there is no lock. See the LOCKS section for more information on the lock information character. The FD column contents constitutes a single field for pars- ing in post-processing scripts.
That is file descriptor.
More on it:
File descriptor (FD) is an abstract indicator for accessing a file. The term is generally used in POSIX operating systems.
In POSIX, a file descriptor is an integer, specifically of the C type int. There are three standard POSIX file descriptors, corresponding to the three standard streams, which presumably every process (save perhaps a daemon) should expect to have.