Basically, if you have made an excess contribution to your HSA, you are permitted to withdraw the excess by the original due date (April 15th in most years) to "undo" the excess contribution.

If you don't withdraw the excess (or all of it), then the amount of excess carried over to next year gets hit with a 6% excess tax and that same amount reduces your contribution limit for next year (in effect).

The phrase "January 1" refers to the first day after the current tax year, up to the due date of the current tax year. This is because you would not be in this situation of excess contributions (or know about it) until after the end of the current tax year when you are doing your current tax year return.

[Edited 3/19/2020 1:40 pm CDT - made year neutral]