A class is said to be instantiated when a concrete occurrence of the class—that is, an instance or object—is created by calling its constructor. Instances on Roblox are created with the Instance.new function, which takes as arguments the name of the class to create an object of and the initial parent of the object.
For example, this code will create a 
Part and add it to the 
Workspace:
local part = Instance.new('Part')
part.Parent = workspace
Services and abstract items (such as an instance with inherited classes) cannot be created with Instance.new, but non-existent services can be created with the game:GetService() function.