Exactly what it sounds like: You're sending a message (usually a NSFW message to your girlfriend/boyfriend) and you realize that you should look before you send - Because that just went to someone whom you REALLY don't want to see it, usually your mother.
Hey babe, my parents aren't home. Let's do it tonight ;)
*5 seconds later*
"OH, SHIT! THAT TEXT WENT TO MOM!"
"You're grounded, young man!"