Mass media are any ways of communicating intended to reach many people. Given technology today, that can mean reaching millions or even billions. Radio, television, newspapers, and various internet sources such as Slate and even Facebook are considered to be mass media. On a smaller scale, newspapers and local television or radio stations might still be considered mass media if they reach thousands. A neighborhood newspaper would not be considered mass media, though. The advantages of time, money, coverage, and impact are powerful in mass media.
Imagine that you want to let everyone in the country know your thoughts about a bill in Congress. You could write a letter to the editor of your local paper and to every other local paper in the United States. You could write in your blog, which has fifty followers, on the bill. Or you could put an advertisement on the local radio stations in fifty states. However, if you post something to Facebook's public, write a letter to the New York Times that gets published, or place an advertisement on a national television network, you will save time and money, you will get national and even international coverage, and you will have a more powerful impact. (While network advertisements are expensive, they still represent a savings over advertising in every single local market in the United States.)
You can see that you can write something once and have it reach thousands, millions, or even billions of people, with maximum coverage and impact, having saved considerable time and money. There are disadvantages to mass media, too, of course, and perhaps that will be a topic covered in your course.