Thanks to everyone who replied to my question. I learned a lot about rendering. As for my original problem, I was seeing a black artifact that flashed on my screen. I saw it when it played in my timeline, and I saw it when I exported to Youtube. In the timeline, it sometimes appeared, and sometimes it didn’t; it wasn’t repeatable. On youtube, it always appeared. I believe the source of it was that I had two video tracks that were supposed to fade in at the same time. One track had a dip to white at the start, and the other hand a crossdissolve at the start. They should have both been dip to white. I think this discrepancy created the problem. When I rendered it, the problem went away. When I changed that both fades to dip to white, the problem went away. Thanks again for your help.