What does the code/function: "Enum" mean?

Recently, I've been following the wiki tutorial as I find it most reliable. However, there were some unclear statements from the wiki which I can't help myself but to ask. I like things to be specific and I'm a curious learner when it comes to computer programming.

In the day an night cycle tutorial: http://wiki.roblox.com/index.php?title=Making_a_Day/Night_Cycle The final peace of code for the lightstreet is this:

lightPart = game.Workspace.LightPart
minutesAfterMidnight = 0
while true do
    minutesAfterMidnight = minutesAfterMidnight + 10
    game.Lighting:SetMinutesAfterMidnight(minutesAfterMidnight)

    if game.Lighting:GetMinutesAfterMidnight() == 6 * 60 then
        lightPart.Material = Enum.Material.Plastic
        lightPart.PointLight.Enabled = false
    end

    wait(0.1)
end

On line 8 of this code: "Enum.Material.Plastic", what does the code "Enum" mean?