There is a full explanation in this post: https://steemit.com/steembasicincome/@steembasicincome/vote-values-in-steem-basic-income-part-four-value-balances-and-upvote-delivery

In brief, the rshares balance shown represents pending upvote value that has not yet been delivered to the account. The rshares balance steadily grows based on subscription level, delegation level, and upvoting rewards.

Each time you post, you can receive up to 20% of your rshares balance as an upvote on that post. We have a minimum upvote of $0.012 to minimize votes going to dust, so that line in question is saying that the balance shown hasn't grown enough to deliver a minimum upvote yet, and the next upvote will come after it does.