I don't know how I can simplify this for you. The debt ceiling is the amount of debt the government is allowed to incure to fund the fiscal year. That includes all it's bills from paychecks for federal employees to interest payments on the national debt. If the debt ceiling isn't raised, the U.S. will then have to decide which expenses and debts to pay and which can wait. You only default if you decide to not pay the interest on the debt.
Ok, so..
debt ceiling is not raised.
Exception is the interest on the debt. We're only allowed to pay that.
The military has ceased getting paychecks and our defense spending is cut off (don't have an exact percentage for you, but a huge portion of federal spending).
Social security and Medicare payments are stopped, people who planned retirement around these programs are left to fend for themselves (don't have an exact percentage for you, but a huge portion of federal spending).
The government grinds to a halt (discretionary spending, a small fraction of federal spending)
In other words, your credit card bills are in a big flaming pile
I'm assuming you're a teenager. It's the only explanation. Talk to your parents about their retirement plan.