Comment by markjdb

It implements the x86 and x86_64 Linux system call ABI. Linux ELF binaries get vectored to an alternate system call table implemented by the compatibility layer. There are some other components like an implementation of a Linux-compatible procfs. How well it works in practice really depends on how far off the beaten path you go. There are lots of non-essential pieces that are not implemented, but for example I know of people running Steam on FreeBSD.