Did you have an HSA? If so, your employer thinks that you are covered by an HDHP (High Deductible Health Plan), probably one that they enrolled you in.
The $3,400 is the annual HSA contribution limit for 2017 for someone covered by a Self-only HDHP plan.
There is a table on the 8889 that lists $3,400 for each month; this is part of the calculation for the annual contribution limit, especially if you were not in the HDHP all year. The calculation is to add all the $3,400 for each month that you were in the HDHP, then divide by 12.
[Edited 3/19/2020 4:13 pm CDT - clarified post]
