The best way to describe as to how to do a pull off on guitar is in a short video, so please check out the short clip I put together for you. Strictly from a technical perspective, a pull off is actually not too complicated. Yet, it is definitely a type of technique that goes a little bit beyond the beginner level. The best way I can explain it: think of two notes you play in sequence. One is higher than the other. While picking the respective guitar string only once, you play the first note and then you remove the finger that plays the higher note. Again, I think when you watch the video it actually becomes pretty clear.
Now, it is important to mention that especially when you are a beginner it might take you a little while to get used to play pull offs on your guitar. The timing is important, especially when you want to make sure that the second notes still has some decent volume to it. I personally found it easier to play pull offs like the first on I demonstrated in the video (when using two fingers), rather than the ones where the second note is an open string.
It’s probably true that guitarists use pull off primarily when playing scales, however, they can also be nicely incorporated in chords and chord progression.