Nostalgebraist is the name of a Tumblr user, whose blog tagline is "Trees are harlequins, words are harlequins" (A quote from a Nabokov novel.)

This gets automatically appended to the page title for anything that gets posted on their Tumblr, hence why it's showing up here.

As for the rest, this is explaining a recently popular neural network architecture known as the Transformer, responsible for advances in natural language processing like GPT-2.