Run-on is a common mistake when 2 independent clauses are not connected with proper punctuation and/or conjugation. This is commonly seen when we write in a fast manner and forgotten to perform a cross-check against the linking sentences.
Here is an example of forgetting to link 2 clauses together with a comma and "and".
We bought nails we bought hammer.
The correct form is:
We bought nails, and we bought hammer.
Here is another example of forgetting to use period to split 2 unrelated independent clauses.
Selina had breakfast she is not hungry.
The correct form is:
Selina had breakfast. Therefore she is not hungry.