Who do box12a mean code D on my W2 it has amount 840.42 in it

[Bsch4477]

That amount is a contribution to your retirement plan. 

[KurtL1]

Code D in Box 12a of your W-2 is indicating that the amount listed is what was withheld from your wages and deposited in 401(k) account.

A 401(k) plan is a company-sponsored retirement account that employees can contribute to, The employee contributions reduce their income taxes for the year they are made, but their withdrawals are taxed.