How do i use the same variable in different scripts?

I dont really understand variable scopes in Roblox Lua so I would like some help with it. When i create a variable, and then try to use it in Another script, i get a error saying "Unknown global"

Script A

var = 3 * 3 --- I set the variable

Script B

print[var] --- I try to use the variable but i get the error.

I would appreciate an explanation on how the scopes work in Roblox Lua and how i make a variable that i can use on all my scripts. Thanks!

###No, don't use _G to make global variables. Use module scripts instead.

-- Place this module in ReplicatedStorage

local module = {}

module.stringthatsayshi = "hi"

function module:printstring()
    print(self.stringthatsayshi)
end

return module 

###From a server/local script:

local module = require(game:GetService("ReplicatedStorage").ModuleScript)

module:printstring()

_G.var = 3*3

This will allow you to use this variable in any script aslong as you put "_G." before it.

print(_G.var) 

Will print 9