Run-on (or fused) sentences are those that use compound sentences without correct punctuation. For example:

Incorrect: I wasn’t busy I was watching TV at home.

Correct: I wasn’t busy; I was watching TV at home.

Correct: I wasn’t busy, but I was watching TV at home.

You can fix run-on sentences by adding proper punctuation (e.g., commas, semicolons, etc.) and coordinating conjunctions. You know these as “and,” “but,” “for,” “or,” “nor,” “so,” and “yet.”