That means certain ways of spelling and using words are embedded in people’s minds, and if you deviate much from the standard, they’ll view your writing as amateurish and sloppy.
You can get it on Amazon for about $11. (I almost said eleven bucks. Guess why I didn’t.)
A few spoilers:
website. Not Web Site or web site
email. Not e-mail
social media. Not Social Media.
Google, Googling, Googled. Google is always capitalized, even when used as a verb.
Get it. Learn it. Use words the way the pros do.