Here’s what to do. Technically it’s a little harder. Don’t enter labels in the text field because as soon as you type they are gone. How do you check you responses when reviewing a completed form?
Imagine you have a placeholder in a date field that tells you the format in which you should enter the date ("mm/dd/yyyy"). You enter the date and that information is no longer visible. You finish the form and think "Wait a second, was that supposed to be mm/dd/yyyy or mm/dd/yy?". That's the general idea.
I believe he's saying to not use "placeholder" attributes to indicate whether a field is required because it disappears when a value is typed into it. Instead, you should use some helper text underneath the field.
Can someone explain that sentence to me?