What is the GetPlayerFromCharacter Function?

The GetPlayerFromCharacter function "returns the Player associated with the given character, or nil if the character is not controlled by a player"

If you have a player's character model (the physical model), you can find the corresponding Player instance for that character (the object in the Players service which has GUIs and the StarterGear and PlayerPack).

The essential is that it "undoes" player.Character, that is,

game.Players:GetPlayerFromCharacter( player.Character ) is player.

Because it returns nil (as opposed to erroring) you can also use this to determine if a given model is actually a ROBLOX player rather than something else in, for example, a Touched event:

-- Example touched event to give a player a weapon
function onTouch(hit)
    if hit.Parent then
        local player = game.Players:GetPlayerFromCharacter(hit.Parent);
        if player then
            game.ReplicatedStorage.Weapon:Clone().Parent = player.PlayerPack;
        end
    end
end
somepart.Touched:connect(onTouch);