It's probably easier to understand if you've used
tee first. This useful old tool takes standard input and writes out to multiple files, plus standard output. The following:
echo "Hello world" | tee one two
Will create two files, named
two, both containing the string
Hello world. It will also print to your terminal.
pee performs a similar function but instead of redirecting output to multiple files it redirects to multiple secondary commands, ala pipes. It differs slightly from
tee in the respect that it doesn't send the original stdin to stdout because it wouldn't make sense combining it with the output of the secondary commands. The following very simple example:
echo "Hello world" | pee cat cat
Will output the string
Hello world to your terminal twice. This is because each of the two instances of
cat receives the standard output and does what
cat does, which is print.