What does run on mean?

A RUN-ON SENTENCE (sometimes called a "fused sentence") has at least two parts, either one of which can stand by itself (in other Words, two independent clauses), but the two parts have been smooshed together instead of being properly connected. Review, also, the section which describes Things That Can Happen Between Two Independent Clauses.

Examples:

-He went to the store he bought a towel.

-You need to go to the store you need clothes