???????

From The LURKMORE Wiki

This joke originated with the comedy South Park in an episode in which the Underpants Gnomes are stealing everyone's underwear. When the main characters ask why the Gnomes are stealing everyone's underwear they show a chart:

1. Take all Underwear
2.  ?
3. PROFIT

None of the gnomes were aware of what the middle step was, though they each assumed that one of the other gnomes did.

Memes

The quote was originally a large single red question mark; in the process of becoming a meme on 4chan it had multiple question marks added to it:

1. Do something
2.  ???????
3. PROFIT