Income tax expense is the corporate income tax recorded for the period, which is not always the same as the tax actually paid. The effective tax rate, found by dividing tax expense by pre-tax income, should usually land near the official rate where the company operates (about 21% in the US after 2018, with state and foreign taxes added on top).
Big swings from year to year hint at one-time items, profits shifted to other countries, or aggressive tax planning. A steadily low rate can be normal for a company that does a lot of business abroad, but a sudden one-year drop usually is not.