Liabilities are obligations of a company arising from past transactions or events which are expected to reduce assets when they are settled.
Note : This is a simple term, but fairly complex topic. A company's balance sheet is made up of three components : Assets, Liabilities, and Shareholder's Equity. Assets are equivalent to the sum of liabilities and shareholder's equity.
Let's say a company sells computers. The assets would be the machinery, the plant, the inventory, etc. the company. However, the value of these assets belong to who funded the company. Is the company backed by obligations/debt (loans from a bank, inventory from a supplier)? or ... The founder / common public's (shareholder's equity)? Most likely, a combination of both.
Simply, liabilities are the part of a company's assets that it has yet to repay back.
Total Liabilities = Total Assets - Shareholders Equity