If you invested $10,000 in an asset and that asset doubled its value to $20,000 - you would make 100% on your investment.
However, assume you invested $10,000 of your money and borrowed $90,000 to buy an asset for $100,000.
If this asset doubled, you would have a gain of $100,000. This is equal to a 1,000% return on your original outlay of $10,000.
You would have leveraged the $10,000 against the total investment of $100,000 in order to make a 1,000% return.
Leveraging magnifies the gains.