I believe they are referring to the Tk GUI toolkit, which is intended for use from Tcl and really nice to work with. It is full featured and has plenty of widgets and layout controllers. You can find out all about Tk here: https://tkdocs.com/

It is really fast and easy to create GUIs using Tcl/Tk, e.g. here is a Tcl script to create a paint-like program in less than 50 lines: https://pastebin.com/6qibnQcJ

The main downside is that the look and feel is not native, and a bit dated. There are some efforts to update the look, but it's not as slick as Gtk etc.

You get both native and custom GUI elements with modern Tk.

And yes, there are plural "layout controllers" (Tk calls them "geometry managers").