A compound sentence joins two sentences using a comma and a connecting word (and, but, or, nor, for, so, yet).
Egypt has about 139 pyramids, and the largest is the Great Pyramid of Giza.
If you leave out the comma and the connecting word, you have an error called a run-on sentence.
Originally it was 480 feet tall the Great Pyramid remained the tallest human-made structure for 3,800 years.
(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.