How do I define "player"?

I'm making a script, and in the output, the only error is that I haven't defined the player, its for a script to equip a tool in a GUI, for a inventory system I made, but the function is : "script.Parent.MouseButton1Click:connect(function(player)", but I need to define player to define the pickaxe. So how would I define the player, here is the code :

local pickaxe = player.Backpack:FindFirstChild("Pickaxe")
--------------------------- "player" definition needed here
script.Parent.MouseButton1Click:connect(function(player)
    local pickaxe = player.Backpack:findFirstChild("Pickaxe")
    if script.Parent.Text == "Unequip" then
        pickaxe.Parent = game.ServerStorage
        script.Parent.Text = "Equip"
    else
        pickaxe:Clone().Parent = player.Backpack
        script.Parent.Text = "Unequip"
    end
end)