What does 'String...' mean?

data class ChatMessage(val dateTime: Int, val userMail: String?, val userMessage: String?) : Comparable<ChatMessage> {
    override fun compareTo(other: ChatMessage) = this.dateTime.compareTo(other.dateTime)
}

private val chatMessages = mutableListOf<ChatMessage>()

data class ChatMessage(val dateTime:Int?, val userMail: String?, val userMessage: String?) : Comparable<ChatMessage> {
    override fun compareTo(other: ChatMessage) = this.dateTime.compareTo(other.dateTime)
}

private val chatMessages = mutableListOf<ChatMessage>()

fun yourCode() {
    newReference.addValueEventListener(object : ValueEventListener {

        /* Use proper variable naming. Nobody will understand, what p0 is, but if you name
        it dataSnapshot, everyone knows at a glance. */
        override fun onDataChange(dataSnapshot: DataSnapshot) {

            chatMessages.clear()

            // Again, what is ds exactly? Name it properly.
            for (ds in dataSnapshot.child(playerIDmatchWhoIs).children) {
                // Kotlin recommends to use val instead of var.
                // This way, you know that your variables cannot be modified unless you want them to be modified.
                val hashMap = ds.getValue() as HashMap<String, String>

                // use indexing instead of the get() method
                val dateTime = hashMap["datetime"]
                val userMail = hashMap["usermail"]
                val userMessage = hashMap["usermessage"]

                // TODO: Handle null values properly
                chatMessages.add(ChatMessage(dateTime!!.toInt(), userMail, userMessage))
                recyclerViewAdapter.notifyDataSetChanged()

            }

            chatMessages.sort()
        }
    })
}