The Tax Cuts and Jobs Act enacted by Congress at the end of 2017, established a whole new set of income tax rates for 2018 and later.
The new rates are somewhat lower than those in effect during 2017 and earlier. As a result, most taxpayers will have to pay less income tax in 2018 than they did in 2017.
If you’re an employee, your employer withholds your income taxes from your paychecks and pays them to the IRS. It’s possible that not as much needs to be withheld during 2018 as in 2017.