A legal interest in property created in favour of someone who is owed money by a person or business, e.g. a mortgage on your own home.
Most of us have property or possessions that we own - a watch, a wedding ring. Some of us have houses which we 'technically' own, but if there is a mortgage or loan outstanding, then the loan provider also has an interest. This external interest is a 'charge' against our ownership - and we cannot sell this property without first satisfying this charge.