If you are not careful, your economy will go into debt. When you have no money left in the bank you go in debt. This can lead to difficulties for your team.
Biggest problem is that you pay 3,33% of your balance as interest every week. Many will have a lot of trouble to pay this interest and at the same time paying off the debt. Especially new clubs should be warned not indebting their club too much. The debt burden if not paid off quickly easily adds up.
If you are € -200,000 or more in debt you cannot buy players from the transfer list and can't replace your coach. Such can seriously impair your progress.
As long as you have a plan to get out of debt, you'll probably be fine. Check your revenue and expenses and figure out whether your revenues in the end will be higher than your expenses. However, make sure you don't end up with a balance of € -500,000 for too long. Two weeks below that number will lead to bankruptcy which will make you lose your team.