p = pull off
h = hammer on
These two things refer to techniques employed by the left hand to sound notes without the direct aid of the right hand. A 'hammer on' is when a note is sounded, and while it is still ringing, a left hand finger is used to quickly press down a fret that is on the same string, and close to the original fret. This will result in the sounding of the second note, even though the right hand did nothing but sound the first note. The vibrations that occurred from the sounding of the first note will be carried on to sound the second note. Here's an example. We can place our index finger on the 3rd string, 5th fret, and sound that note. While the note is still sounding, and without moving our index finger, we can use our ring finger to come firmly and directly down on the 7th fret of the 3rd string. This note will then sound, probably not as loud as the first note, but it will still sound. This is a hammer on.
A 'pull off' is basically the same thing, but backwards. We can use a finger to fret a note, and then sound that note. While the note is still sounding, a finger can be placed directly behind the first finger, and then the first finger can be 'pulled off'. In doing this, it is in effect the same as plucking the string with a finger, it creates vibrations. And now the note that is being fretted directly behind the first finger will be sounded. For example, we could place our index finger on the 5th fret, 3rd string AND place our ring finger on the 7th fret, 3rd string. We can play the note on the 7th fret, and then 'pull' that finger off the string, which at the same time, will sound the note on the 5th fret. This is a pull off.