Why is 'player' read as a number?

local swords = game.ServerStorage.Swords
local swordschildren = swords:GetChildren()
local secondaries = {swordschildren}

function equip(player)
	local stats = player.stats
	local backpackitems = player.Backpack:GetChildren()
	local swords = game.ServerStorage.Swords
	for i,v in pairs(backpackitems) do
		if v == secondaries then
			v:Delete()
			local itemclone = swords[script.Parent.Parent.ItemName.Text]:Clone()
			itemclone = player.Backpack
		end
	end
end

script.Parent.MouseButton1Down:connect(equip)

ERROR: 21:38:40.568 - Players.Player1.PlayerGui.ScreenGui.Shop.Pages.Inventory.ItemDetails.Equip.Script:6: attempt to index local 'player' (a number value) 21:38:40.569 - Stack Begin 21:38:40.569 - Script 'Players.Player1.PlayerGui.ScreenGui.Shop.Pages.Inventory.ItemDetails.Equip.Script', Line 6

When I printed the number which it thought player was, it simply printed the number '1078'. I'm very confused and tired. Your help is much appreciated.

The error says that player is a number, so you should believe that.

Where is player defined? On line 5, as the parameter to equip.

How does equip get called? By the MouseButton1Down event (of presumably a GuiButton).

Let's check MouseButton1Down on the Wiki.

The event says it passes two numbers, an x and a y, presumably the coordinates of the click.

Not the player who clicked.

Why would you expect it to be a Player object if the documentation doesn't suggest that?

GUI events like clicking should not be done in a Script by the server; they should be done in a LocalScript and communicated to the server via RemoteEvents.

This Wiki article walks through a very simple example (almost exactly what you are doing) of properly splitting your logic between LocalScripts and Scripts.