Whats the third paremeter of pcall and what does it do?

I believe this is a snippet from the a MarketplaceService article. This third argument isn't specific to a Player Object, it actually represents the argument being supplied into the function defined in the first.

This is a bit tough to grasp, however, let me try my best to explain...

The most common representations of pcalls usually just receive a fresh function, of which we use to perform our instructions, E.g:

local Success, Response = pcall(function()
    print("Do something")
end)

If you're unfamiliar with what this first argument is, the example above should be quite obvious, pcall() will take the function provided and run it in what we call a try except block within other coding languages. Essentially, it will operate the instructions in a safe environment, meaning any potential exceptions thrown will not interrupt the Lua thread.

However, with your case above, you're using a more advanced form of a pcall() use. The structure holds itself like so: Function, Function Family; where it belongs to, & the argument that will by supplied when running it.

Let's take a look at an example of this with a DataStore, that way, we can make it a little easier to understand:

Note:

If there are no issues, Response is the variable :GetAsync() will return to.

local DataStoreService = game:GetService("DataStoreService")
local DataStore = DataStoreService:GetDataStore("MyData")

local Players = game:GetService("Players")

local Data;

local function GetData(Player)
    local Success, Response = pcall(
        DataStore.GetAsync, --// Function
        DataStore, --// Belongs to our DataStore Object
        Player.UserId --// Argument supplied
    )
    if (Success) then Data = Response end
end

Players.PlayerAdded:Connect(GetData)

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