What is “the polyclonal antibody response of a highly neutralizing COVID-19 convalescent plasma” in less technical terms?
Highly neutralizing -> very effective at murdering COVID virus
Polyclonal antibody response -> the person's immune system produced different antibodies to fight COVID which probably attack different parts of the virus
Basically, you take the blood plasma of someone recently recovered from COVID-19 with high antibody counts and filter antibodies out of it, that are then administered to critically ill patients. Same procedure is used in manufacturing snake venom antidotes (with horses as host bodies, since they are able to resist snake bites). More about this can be read e.g. here: https://www.mayoclinic.org/tests-procedures/convalescent-pla...
As for the different type of antibody - tl;dr of https://www.ptglab.com/news/blog/polyclonal-vs-monoclonal-an... is "monoclonal antibodies are all of the same configuration and target one specific molecule, whereas polyclonal antibodies have a higher amount of variance in matching".
As for "what exactly did they do" - they took samples of the specific virus and exposed it to such a blood plasma sample, and noticed escape, aka surviving infectious viruses. Which in turn is bad news as it means that this specific strain of virus at least has the potential capability to evade the body's immune system protection that has been built up by infection or immunization.
As this is an "in vitro" test though, this means that it may also be possible that an "in vivo" test (=exposing a live human instead of a blood sample) would fail.
It's been used in Ebola outbreaks as a "hail mary" to cure people - it's also one of the more absurdly easy things to do - pretty much just take a blood sample, spin out the platelets and inject the plasma left behind (clinically you can be a lot more careful, but its one of the weirdest effective "primitive" treatments you can do).