When you join two independent clauses with a comma and no conjunction, it’s called a comma splice. Some people consider this a type of run-on sentence, while other people think of it as a punctuation error.
Here’s an example of a comma splice: Koala bears are not actually bears, they are marsupials.
There are three ways to fix a comma splice. You can add a conjunction, change the comma to a semicolon, or make each independent clause its own sentence.