Hace mu literally means "makes moo". In Spanish, the verb hacer (to make or to do) is used to describe the sounds animals make. This is different from English where we typically say an animal "says" or "goes" moo.
This simple but essential phrase is the heart of the classic children's song, "La Vaca Lola". Learning it will not only help you sing along but also teach you how to talk about all your favorite animal sounds in Spanish!
This song is a super fun and simple tune all about a cow named Lola! The lyrics introduce her with a very basic, almost silly description: "La vaca Lola, la vaca Lola / Tiene cabeza y tiene cola" (Lola the cow, Lola the cow / She has a head and she has a tail). The main point of the song is to teach you the sound a cow makes, which is, of course, a big "Muuuuu"!
After we meet Lola, the song takes us to a farm, specifically "En la granja de mi tío" (On my uncle's farm), which you might recognize from the English song "Old MacDonald Had a Farm." Here, we meet more animals and learn their sounds. There's a cat that goes "miau", a duck that goes "cuak", and a goat that goes "Beeeh". It's a great way to learn the names of different farm animals and the noises they make in Spanish.