Proofreading is simply catching spelling, punctuation, capitalization, and formatting errors, not usage errors, which would involve editing. One of the most common punctuation errors is the comma splice, sometimes called the comma fault. You will not see this error in any first rate publication or website. Here is an example of a comma splice from grammarly.com.
Incorrect: Koala bears are not actual bears, they are marsupials.
Correct: Koala bears are not actual bears. They are marsupials.
There are three ways to correct a comma splice: one, use a period (.); two, use a conjunction (and, but, or); and three, use a semi-colon (;). A semi-colon connects two sentences which are closely related. However, use a semi-colon sparingly.