负债 is a verb-object compound (离合词) meaning 'to be in debt' or 'to incur debt'. The object 债 can be separated from the verb 负 — for example, 负了债 (incurred debt) or 负过债 (have been in debt). You can also insert a measure word or modifier: 负了很多债 (incurred a lot of debt). It can also function as a noun in financial contexts (e.g., 公司负债 'company liabilities'), but the primary use is as a verb-object.