bash じゃないと無理なのねorz

:() defines a function named : with no arguments.
{ :|:& } is what the function does.
:|: calls itself twice (with a pipe between the two), and the & at the end runs it in the background as a new process.
The ; finishes off that command, then the last : runs the function, starting the fork bomb
(as each run starts 2 new processes, each of which starts 2 new processes...).