Levering is where you borrow to invest.
For an investment property you could borrow perhaps 70% of the value of the property. If the property goes up by 10% in value because you only put down 30% the real return to you is 33%
10,000 increase on an investment of 30,000.
This is leveraging and does not mean leveraging your own debt. It is also often referred to as OPM ‘Other People’s money’ or gearing.