Serial to Parallel Converter
So, I myself have taught based on material that uses that scheme, "P/S" and "S/P" after and before the transforms.

Personally, it's nonsense.

What the author tries to say is:


  The IFFT is a mapping of sample vectors to vectors. So, you need a vector as input, not a stream of samples.


What they instead say is:


  We use the terminology from very basic digital logic to deal with complex values.


That's not only a bit awkward, if you asked me, but also inaccurate:


modern FFT implementations in hardware actually do take in stream data, so that operation isn't there in practical hardware implementations
in software implementations, you basically never even deal with samples coming in serially – they always appear en block, in some memory location, so nothing's "serial" to even begin with
S/P nor P/S even say that you should be fully accumulating one vector of $N$ samples, then move on to the next vector, and have zero overlap between these – when applied to FIFOs (which is where the term comes from), that's usually not how they work.


So, simply think of S/P as "get $N$ samples, present these $N$ to the next block as vector". 

And P/S is "take this $N$ long vector, and give one sample after the other".

So, these blocks do exactly nothing to your signal - it's just a reinterpretation, if you will, between things that are logically "one after the other", and things that are logically "a vector of $N$ values".