You may have heard that President Donald Trump is sick with COVID-19. He immediately received an experimental "polyclonal antibody" cocktail made by the large biopharma company Regeneron. This treatment has not yet been evaluated by the FDA.
When our immune systems begin generating antibodies to a foreign object, they don't generate just one kind of antibody. They generate many, many different kinds, that have different structures and also target different individual pieces of the thing it's attacking (through a genetic mechanism unique to the immune system, immune cells can shuffle the genes that code for a "variable" region of an antibody like a deck of cards, generating antibodies of variable shape and targeting). So, in the case of SARS-CoV-2, our immune systems might generate one antibody that attacks the spike protein, while another antibody that attacks a part of the viral envelope. Or, they might generate multiple antibodies that all attack different parts of the spike protein.
To create this antibody cocktail, Regeneron created "humanized" mice, which had immune systems more closely related to the human version than their natural mouse immune system. Sifting through antibodies from these mice after infection with SARS-CoV-2 and antibodies from humans who'd recovered from infection, they isolated two antibodies that targeted two different parts of the virus.
It's called "polyclonal" because it's a mix of genetically distinct antibodies (with different targets). A collection of genetically identical antibodies that all attacked the same thing would be called "monoclonal." Whether this truly "polyclonal" or "two monoclonal antibodies mixed together" is a somewhat academic debate, in my mind.
The idea is that the antibodies shouldn't block each other out (imagine two people reaching for the same doorknob) and will hopefully have a combinatorial effect. The cocktail is still in trials but has some positive data.