An overview is a general summary of something. An overview gives the big picture, while leaving out the minor details.
If you take a class on American history, the class will be an overview: it will sum up the most important events that have happened, such as famous elections, national disasters, major wars, and economic trends. When writing an overview, you have to leave out a lot of details, because you're focusing on major points. You can give an overview of a company's business, of a sport team's season, or even of your own life.