This distribution from the 401(k) represents money that you deferred from your paycheck so that you would not have to pay taxes on it at the time that you earned it, so now this money and the investment earnings on this money are taxable since they've been distributed to you (and presumably not rolled over. Box 2a shows the taxable amount (likely the entire amount shown in box 1 and the amount in box 4 is what ws withheld for taxes and will be credited on Form 1040 line 25b. Box 2b Total distribution being marked simply means that this distribution closed out the 401(k) account.
Enter the Form 1099-R exactly as received under Wages & Income (or Personal Income) -> Retirement Plans and Social Security -> IRA, 401(k), Pension Plan Withdrawals (1099-R) and answer the questions that follow. Assuming that you did not move the money to another retirement account, the box 2a amount will be included in income of Form 1040 line 5b and you'll also have a 10% early-distribution penalty on Schedule 2 line 6.
A 401(k) is a qualified retirement plan.
