Understanding sentence boundaries is essential to writing. A sentence (at its most basic) requires a subject (a noun) and a verb (an action).
Example: He ran.
The subject here (he) did an action (ran) and so we have a complete sentence. If you put two complete sentences together, then you have a run-on sentence.
Run-On: He ran the store was busy.
“He ran the store” could be a complete sentence as could “He ran” and “The store was crowded.” We have two subjects and two verbs and it doesn’t make sense when they are jammed together like that. There are a few ways that we can fix this.
Separate the sentences: He ran. The store was busy.
Add a conjunction: He ran because the store was busy.
Add a conjunction and a pronoun: He ran the store and it was busy.
If you aren’t sure if your sentence is a run-on or you know that it’s a run-on, but aren’t sure how to fix it, try finding the verbs and the nouns that they go with. If you have two things doing two actions, then you probably have a run-on sentence.