In this video
Hammer ons are when a note is played and then a second note is sounded by hammering down a finger on the same string at a higher fret.
Flick offs aka pull offs are the reverse of this. The higher note is picked and then flicked off to a lower note.
Here's a basic Hammer Ons and Flick offs exercise to get you started.
Here's the riff that I play at the beginning of the video. This is a great example of how this technique can be integrated with chords and rhythm to create a more fun and engaging sound.
Interactive TAB