This value shows how much budget your club will have when the next season begins. If this value is negative then your club has more forecasted expenses than income. If this is the case then you should try your best to get out of the red (sell players, get a sponsor, investor...) or to reduce your costs. If you don't balance the books by the beginning of the season then you run the risk of the board selling your best players to cover the deficit.

Be aware that this value is an estimate based on your current situation. Unforeseen changes like your club being relegated are not taken into account.