A chord is a set of three (or more) notes played at the same time. These distinct notes have a special relation to each other which makes them sound good to our ears.
For now, this basic definition is all you need. If you want to get geeky, you can check out my theory article.
You will also see, that most of the basic chords and riffs in this lesson do not require an uber-human skill. Yet, they sound killer!