An Alfredo sauce is a pasta sauce, made with Parmesan cheese, butter and garlic. It was named by an Italian restaurateur at his restaurant Alfredo alla Scrofa in Rome.
There are many ways to prepare this tasty sauce, but the basic parts are cream, butter and Parmesan cheese.
You can add other ingredients as well to an Alfredo sauce, like parsley and Italian spices.
Here's a video on how to make a penne pasta with Alfredo sauce, step-by-step: