What is a comma splice?

 

I remember grammar lessons at junior school but not in great detail.

I usually know when a sentence is badly structured but it is a rare moment when I can actually explain "why" it sounds so wrong.

Basically all grammar makes for clear communication. Commas  are necessary to avoid confusion in a long sentence. My nearest and dearest often runs two separate sentences together, as in "She sleeps soundly, he lies awake and reads." This is a comma splice and Emma Darwin describes it perfectly:

"When two grammatically complete sentences, which really need a full stop between them, are separated by a mere comma, the grammar and syntax of the first sentence lead to the finish, but the comma isn't a firm enough stop. So the first sentence seems to tumble on, over the break, and land on the beginning of the second – which then doesn't get to launch properly. The paragraph as a whole feels sloppy and incoherent, as reading aloud will reveal, even if your eye didn't spot it.”

"She sleeps soundly; he lies awake and reads."

In this case a simple colon provides the slight pause required to separate two independent but closely connected sentences. There are lots of other examples, and you can read a clear explanation of the Comma Splice on the blog: https://emmadarwin.typepad.com/thisitchofwriting/itchy-bite-sized/