The d command runs a command in the background and redirects its output to a file. The output file is annotated with start and end time, the actual command used, cwd, host, etc. You can ask the d command to extract the last command from the output file and run it again, you can ask it to append to the file or not, and other goodies. It comes with handy little programs to look at the output, tail it, etc. It is sort of trivial, but has been streamlined over many years.