A compound sentence joins two sentences using a comma and a connecting word (and, but, or, nor, for, so, yet).
I want to be a park ranger, and Yellowstone is my favorite park.
If you leave out the comma and the connecting word, you have an error called a run-on sentence.
Yellowstone has geysers a supervolcano lies under it.
(It's hard to tell where the first idea stops and the next idea starts.)
Fix a run-on by adding a comma and connecting word.