It means that the IRS should be allowed to complete your original return BEFORE you file the amended one, however if you will owe additional tax, (or will need to pay back some of your refund) make sure you do that by April 15th.
The IRS may adjust your original return for the missing W-2, in which case you will get a letter and the chance to agree with their change.
If they don't adjust for the missing W-2, you still want to wait or the returns will cross in the system and cause confusion.
If it gets close to April 15th, calculate the amended return (1040X) and mail the additional tax due with the memo 2018 tax year and your social security number on the check, (or pay it online).
https://ttlc.intuit.com/replies/3288565
