What does 'plugin' do for plugins?

I was reading the wiki pages for plugin, and I found this.

local hasLoaded = plugin:GetSetting("pluginHasLoaded")

I know it works, but the script analysis keeps bugging me saying unknown global 'plugin'.

Can plugin be anything?

local hasLoaded = PLUGIN:GetSetting("pluginHasLoaded")
local hasLoaded = superCalaFragaListicExpyAladocious:GetSetting("pluginHasLoaded")

I'm just guessing that wouldn't work, but why? When the plugin runs, does it make a new thing like.

plugin = game.ROBLOXSecretJunk:MakeAPluginHappen("ÜberQuick")

And another thing, if plugin is an unknown global, doesn't that mean you can't test plugins without restarting studio? I mean you have to update the plugin every time you want to fix it, and then close studio, and then you can check to see if your update fixed the issue, BUT NO! You then repeat that 20 times until you fix it...

So, what does plugin mean? And can you test plugins in studio without all the hassle of restarting studio?