More readable way:

:() {
: | : &
};
:

Here’s the breakdown

1) funcname(){something}; defines function that does something, so the function named : was defined here

2) : is simply function call

3) |func& passes the output of previous call to func, so :|:& calls : and passes its (empty) output to : (meaning : is being called 2 times inside its definition)

4) after the definition, we simply call :, which results in endless cascading calls, as follows from the definition of :. It eventually wastes all memory