Here's an example of a short run-on sentence: I'm a woman I am a truck driver. (I was in a writing group a few years ago with an interesting woman who was writing a book about her experience as a female truck driver.) The reason I am a woman I am a truck driver is a run-on sentence is that it's written without any internal punctuation. I've fused together two complete sentences, which is why run-on sentences are also called fused sentences.
There are a bunch of ways to fix run-on sentences; the toolbox is filled with the same basic fixes* that you can use to repair comma splices: periods, semicolons, and commas with coordinating conjunctions.