What does this mean?

Help and Feedback Scripting Support
1

I’ve seen this in scripts, but I don’t know what it means or how to use it. I’d like to know what this means, so maybe I can use it for future scripts I’ll write.

local Gui:ScreenGui = script:WaitForChild("LocalGui")
local GuiTowerRushProgress:TextLabel = Gui:FindFirstChild("TowerRushProgress",true)
2

They are types, which is something that Luau supports (see Type checking - Luau).