What's a ModuleScript

Modules

ModuleScripts are essential when trying to avoid using the same functions over and over again. Functions, tables, and just about any variable set in a ModuleScript can be used in multiple scripts without the need to re-write them all over again.

Modules scripts only run once no matter how many times called in a server, therefore, the results will never change. When called from LocalScripts, however, it runs once for each individual client that calls require.

Require

require comes into play because ModuleScripts do not automatically run when the server initiates, but rather waits until required, and then executes, returning whatever it was set to return.

Module script

Note that you don't have to return tables, but could return just about anything

local module = {
    [1] = 'Scripting';
    [2] = 'Helpers';
};

return module

Server or Client Script

local module = require(['path to module'])
print(module[1], module[2])

AssetIds

Also, requiring modules upload to the ROBLOX website is also possible as long as the module you require is named MainModule (not the model, the instance at time of saving). It can be load using its assetId

require(1)

Example

MainModule

local debris = game:GetService('Debris')

local function createMessage (message, time)    
    local screen = Instance.new('Message')
    screen.Text = tostring(message)
    screen.Parent = workspace

    debris:AddItem(screen, time)
end

return createMessage;

Server or Client Script

Note: if the module only returns a function, you can save time by doing something like require(id)(arguments) instead of *local module = require(id); module(arguments)

require(id)('Scripting Helpers', 5)
--[[ would create a message in the workspace 
with the text `ScriptingHelpers`, and will
only display for five seconds as indicated
by the arguments.
]]

require(game.Workspace.ModuleScript)

You use this to access variables inside of modules scripts for example, this is a module script in workspace

local module = {}

module.Variable = "hi"

return module

now in a normal server script

local modulescript = require(game.Workspace.Module)
print(modulescript.Variable)

hi

You can use this to store functions and a lot of stuff, In short, require gets data from a module script