In addition to the great answer of yole, the documentation is pretty clear as well:
Note that parameters of the primary constructor can be used in the initializer blocks. They can also be used in property initializers declared in the class body. [...] In fact, for declaring properties and initializing them from the primary constructor, Kotlin has a concise syntax:
class Person(val firstName: String, val lastName: String, var age: Int) { // ...}Much the same way as regular properties, the properties declared in the primary constructor can be mutable (var) or read-only (val).
This all does not apply to secondary constructors.