"Muletillas" is just the Spanish word for the general linguistic phenomenon of fillers. Both "muletillas" and "fillers" mean the sam thing and serve the same purpose, for example to inform your listener that you're still thinking and forming a sentence, that you are not done.

Now I want to read this book about fillers that I just stumbled upon:

https://benjamins.com/#catalog/books/tsl.93/main