(String..String?) represents a flexible type with lower bound String and upperbound String? (nullable string). This is not valid Kotlin code (it's not denotable) but it is used in the compiler internals and thus in IntelliJ's hints sometimes.

(On the JVM we often see platform types using ! as in String!, which are a more specific case of flexible types)

It's Kotlin's way of saying it doesn't know whether the String type declared for payload.email is nullable or not (for instance if this is declared in Java, which doesn't distinguish those), and yet it doesn't want to enforce either of those, for convenience (hence "flexible").

As the name suggests, flexible types are flexible — a value of type (L..U) can be used in any context, where one of the possible types between L and U is needed

This means that even though the actual type of the value is "somewhere between String and String?", values of this type can be used even in places expecting String, even though the real type of that value may be String? and thus the value could be null.

This is useful because assuming it is String would mean that null checks would be marked as redundant, and assuming it is String? would force the developer to write null checks everywhere, even though they might know that this particular Java method cannot return null.

In general, it's a good practice to explicitly declare the type of a variable that you get from Java, to avoid the propagation of the platform type and the uncertainty (and unsafety) that comes with it:

val email: String = payload.email // if you know it cannot be null
val email: String? = payload.email // if you don't know