Inbox functionality is straightforward but somewhat unclear; I had to experiment with it in order to actually understand how it works.
The "instructions" disappear as soon as you create a card in your inbox. Might be better for them to be visible permanently in a sidebar.
They read:
"You can create cards here to capture thoughts you don't yet have a stream for or if you're not sure what stream to put the thought in."
Additional information would be handy here: