If you grew up in the fifties, we are probably much of an age. Nobody taught me this stuff at school, either.
The point with computers is that they are complex, counter-intuitive for many people, and fundamentally stupid. Learning to deal with them is like learning to speak a foreign language; no one can do it for you, so you just have to do it for yourself by fitting the pieces of knowledge together until you have something more like a completed jigsaw.
The bit of the jigsaw that is Eraser is not all that complex. Like most people our age, you have probably used a tape or cassette recorder. Eraser works like the erase on such machines, by over-writing what was there before. You do this for the same reason you would shred confidential paper documents - to make sure that information you no longer need but want to keep private cannot be recovered by someone else. When Windows 'deletes' a file, all it does is mark the space as available for re-use; it doesn't actually remove the information from your hard drive.
How do you use Eraser? That's where my note about getting to know the program comes in. I've looked at it again, and for anyone who has even a rudimentary knowledge of Windows, it should be clear enough. If there is anything specific in the note you don't understand, do ask about that. But please stop trying to convince yourself that it's all above your head. It isn't really. Honest.