Since people are downvoting, let me explain with an example to make things clear:
Fractional-reserve banking is the common practice by commercial banks of accepting deposits, and making loans or investments, while holding reserves at least equal to a fraction of the bank's deposit liabilities.
Let's say we have a 5% reserve, which is realistic if you look at EU and US. Let's say there is a new bank, and you deposit 100 euro's at that bank.
I go there to lend out $95, which is allowed because the bank keeps $5. You have a nice shop, and I buy something for $95.
You deposit this $95 at the bank, which they can lend out again keeping a 5% fraction. I go there and lend out $90, buy something at your shop, and you again deposit this at the bank. We do the same with $85 and $80.
So you now have an account with 100 + 95 + 90 + 85 + 80 = $450. Nice! Now you go to that bank and request that money in cash.... oops.
Seems weird, but this is actually how it works. Look it up if you don't believe me: fractional reserve banking!!!