They're called "Lambda Diagrams". See the inventor's homepage [1], and some animations [2]. There's actually a long and obscure history of visualizations for lambda calculus. One of the originals is "To Dissect a Mockingbird" [3], which if I recall correctly, was the main thing on the internet in 1996.

[1] https://tromp.github.io/ [2] https://www.youtube.com/playlist?list=PLi8_XqluS5xc7GL-bgVrx...

[3] https://dkeenan.com/Lambda/