Parameter for function question?

Yes.

I'd assume, from the wording of your question, that you're asking this regarding the ROBLOX wiki. I'll give you an example of how this works.

game.Players:GetPlayerFromCharacter(character)

This is a simple function to use a player's character to get a player object. The "character" variable is what we use to find this player.

For example,

local char = workspace.SystemUnderflow
local plr = game.Players:GetPlayerFromCharacter(char)

The name "char" or "plr" is just shorthand for "character" and "player".

When you give the function a variable that is not correct, then it will either error or return nil. So if you try to use a Player object instead of a character model, it'll return nil.