That's it. The selected conversations are archived.
And, here's an important thing to note: The archived conversation will be automatically unarchived if it receives a message or you send a message from it.
To view archived conversations: Hit the Overflow menu (three vertical dots) at the top and then tap "Archived." Now, you can see the conversations you've archived.
To unarchive: Select the archived conversations. Hit the "Unarchive" icon at the top.