The XRP Ledger has something called "reserves": a certain amount of XRP is "locked up" by the XRP Ledger for account ownership & object ownership (like Trust Lines, to hold tokens, Escrows, Offers, etc.). Read more about reserves here:
It is possible to be below the reserve requirement: the XRP Ledger does not enforce the reserve for the first two Trust Lines created (tokens held) and the reserve can be spent on on ledger fees. If this happens, your account reserve can result in a negative spendable balance.
As XUMM shows the spendable amount of XRP you hold, instead of your raw XRP amount including locked up amounts, XUMM then shows a negative spendable balance.