This is Lana from Shopify.
You are correct - the Balance tab shows what is due in your next payout plus and funds that are held in reserve. These are funds that you are yet due to receive.
There are times when you may see a negative balance, this occurs in cases where after you already received all funds in the previous balance, you make a sale that then has to be refunded. After that, once you have the negative balance you might not be able to issue a refund. Refunds are listed as Pending until you have made enough sales to have a positive balance in your Shopify Payments account, and then you can successfully issue a refund.
For more detailed information on Shopify Payments and the Payouts section, please see our help doc on the topic here.
If you're ever stuck for the meaning of any of the sections within your Shopify admin and you need a quick answer, make sure to check out our Shopify Help Centre. Here you can search for all things Shopify and get answers for questions such as what we were looking at here with the Balance, or even step by step tutorials and guides on all the different aspects of Shopify.
If you have any other questions, please feel free to reach out to me here.