An un-zonked type can have type variables which are mutable references filled in during unification (and this mutability is heavily used by the type checker to increase performance). Zonking traverses a type and replaces all mutable references with the type that they dereference to; thus, the resulting structure is immutable and requires no dereferencing to interpret.
Note that these type variables are meta-variables, i.e. they don’t correspond to the type variables introduced by polymorphism; rather, they are unification variables to be replaced by real types. The choice of replacement is decided by the type checking/type inference process, and then the actual replacement is done during zonking.
This notion of zonking extends naturally to other intermediate representations of the typechecker that contain types.