It is found by taking revenue and subtracting the costs of running the business, such as the cost of goods or services, salaries, research, marketing and depreciation. It leaves out interest on debt, taxes and usually one-off items that are not part of regular operations, such as investment gains.
That makes it a cleaner view of how well the main business performs. Two companies with different debt levels or tax situations can be compared more fairly at this line than at net income. It is also called operating profit or EBIT in many cases, although the exact definitions differ.
It has weaknesses. A company can shift costs between lines, and a big one-time charge inside operations can distort a single quarter. It also does not show cash, since it includes non-cash costs such as depreciation.
Track it over time, compare it with revenue through operating margin, and read the notes to see if unusual items are included.