:(){ :|:& };: fork bomb

:(){ :|:& };:

In fact what you posted there is called a fork bomb, because it does exactly that it is a bash function which calls itself twice and so on. You can replace the : with a name if you want so it becomes more obvious:

fu{
    fu | fu &
}; fu

So fucalls itself piping its output through itself again and this way fills up your processor with requests.

It is not harmful, beside the fact that you may have to reboot your computer the hard way because it becomes unresponsive.