The X in box 13 Retirement plan of your husband's 2018 W-2 indicates that additions were made to your husband's workplace retirement plan for the plan year ending in 2018. Even if your husband made no elective deferrals or Roth contributions to the employer's plan, additions can result from employer contributions or forfeitures by other employees of unvested portions of employer contributions were deposited into this account. Workplace retirement coverage can also result if the employer provides a pension plan.
If no additions were made for the plan year ending in 2018, box 13 Retirement plan should not be marked. Plan can have a fiscal year rather than a calendar year, so it can be a bit tricky to match up the timing of additions with the individual's tax year.
The marking of box 13 Retirement plan only potentially affects deductibility of traditional IRA contributions.