This simply means that “everyday” is used to describe or modify a noun, while “every day” is a phrase used to describe or modify a verb. For example:
If you change them to “an every day outfit” or “ran 4 miles everyday last week,” the sentences no longer mean the same thing or, in most cases, even make sense.
They cannot and should not be used interchangeably.
This concludes my grammar rant for the day. Commence overreaction.