It loosely follows the same metaphor as the T-shaped pipe connector that 'tee' is named for.

    $ seq 3 | tee somefile
    1
    2
    3

            ╔═somefile
    seq═tee═╣    
            ╚═stdout


    $ seq 10 | pee 'head -n 3' 'tail -n 3'
    1
    2
    3
    8 
    9
    10


            ╔═head═╗
    seq═pee═╣      ╠═stdout
            ╚═tail═╝

I believe it's more like `a | pee b x y z | c`. This would run `b x y x` as well as `c` using the output from `a`.

I think Zsh supports this natively with its "multios" option, `a > >(b c y z) > >(c)`. But then you have to write the rest of your pipe inside the ().

I can only guess from that description is that it is the cat command.