Agda uses the notation Set for the first universe. This is incorrect from the point of view of Homotopy type theory, as not all types in this universe are sets. Following HoTTAgda, we shall denote this instead as Type. We accordingly modify the definition of Booleans.
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The statement Bool : Type should be read as saying that Bool has type Type, which implies in particular that it is a type. This is analogous to an expression like 3 : Int in a programming language, which says that 3 is an integer.
Note that Type itself has to have type in a higher universe to avoid Russel’s paradox. Agda (like HoTT) has a hierarchy of such universes, $Set = Set_0$, $Set_1$ etc.