well first off,

they are called "Hammer ons" and "pull offs" a very simple thing to do, but hard to master. What you basically do is from open just push down on the fret and it will make its tone. Remember the basics, use the BOTTOM of the fret or else you will get a buzzy sound. Then Just cleanly pull your finger off of the note and it will make a open sound. This is the most basic hammer on and pull off exercise. once you get more complex you will be hammering on in a middle of a riff and pulling off. Which is something ill leave another to explain.

a simple exercise: hold the G string down at the 2nd fret with your index finger. pick the string and then very slightly bend the string up and then let it go. it will kind snap. what you are doing here is a pull off whereby you are pulling your finger off of the string to the open string. a hammer on is kind of the opposite of that exercise. strike the open G string then slam your index finger down on the 2nd fret and repeat in a quick fashion. once you master that you can experiment with hammering an additional higher fret along with the first hits creating a three note hammer-on progression. additional notes can be added with other fingers to creat even more complex hammer-on patterns....