Castúo is a way of talking that people use in some parts of Spain, especially in the region of Extremadura. It's like a different version of Spanish, with its own words and sounds.
Imagine you have two kinds of toys: one set of blocks and another set of toy animals. Castúo is like the animal toys - they look different and have different names, but they kind of do the same thing as the block toys. So when people speak Castúo, they use different words than in regular Spanish, but they still can understand each other and communicate.
For example, in Castúo they might say "bitis" instead of "víbora" (viper), or "jarabuza" instead of "almendra" (almond). It's kind of like a secret language that people who live in that area can use to talk to each other and feel special.