The comic is about grammar, so go ahead; read it and chuckle. My lesson, though, is in the last panel.
We call it direct address when you name the person (or persons—or readers, shall we say) whom you are writing to. Sometimes I start a post with, “Our lesson for today, class, is…” That’s direct address.
Here’s the rule, in case the rule isn’t already obvious:
Separate direct address from the rest of the sentence with commas.
Of course, the plural, “commas,” applies only if the direct address is inside the sentence. If it’s the first or last word, you need only one comma. duh.
PS—wouldn’t you know, the comic repeated the same joke a week later: