What is the difference between resonance and π conjugation
You might be confused but not missing anything big or fundamental.
A pi-conjugated system is one in which delocalisation of pi-electrons can take place. It is all.
When effective, pi-conjugation is a subset, or a type, of delocalisation than can be described by resonance.
However:
to be strict, conjugation refers to bonds, and specifically to an alternating single and double bonds structure. It is just a positional nomenclature.
It turns out that in cases and in different extent the structure with such conjugated alternating single and double bonds does not represent the real molecule right because delocalisation takes place.
While the terms are linked, a not very accurate use of them leads to their treatment as if they are synonyms. That is what I have more or less done above, but just using a more nuanced term as pi-conjugation, thereby referring to electrons rather than bonds.
This is as to say that a sequence of alternating single and double bonds (as in at least one limiting form) is conjugated by definition. The conjugation (would be better saying delocalisation, here) can eventually/effectively be so true to the point of erasing the original meaning, i.e. in fact there are even not localized conjugated bonds.
Whit all the above in mind, a perfect organic metal with equalised bonds would be seen as &quot;the king&quot; of conjugated polymers.
Finally, within the context and dictionary of  VB the above delocalisation is described by resonance, This again lead to the use of delocalisation and resonance as interchangeable terms.