The single colon indicates the start of the initialiser list. Like so:
Constructorname(parameters) : namespace::baseConstrustorname(baseconstructorParameters)
{
body
}
the syntax is similar to how you declare an inherited class:
classname : public baseclass
But they only work for constructors, base constructors and member variables.
What you're doing is passing your argument title, to the normal RenderWindow constructor, that way all the data that Renderwindow normally takes care of(which you also inherited) is still initialised properly.
Look up "initialiser lists" for more info, you can do other stuff with them as well, such as initialising variables more efficiently.