Well if you ever do monthly installments when signing players then that all adds up to the months.
Player & staff wages tend to do a lot of damages to your finances.
Profits can be made through TV revenue, Merchandise (Especially if your club plays abroad, which expands your fan base), Tickets sold, Competition bonuses.
I don't know much else since I rarely ever go in to debt since I keep a close eye on my finances and make sure i have enough coming in to cover the amount going out.