A hammer-on is when you tap the fret so it makes a noise without
strumming the string with your left hand.
In "Thunderstruck" you also do pull-offs, which is when you
just release the finger, again without using your right hand.
In the case of "Thunderstruck", you have to put some tension on the
string before you release it, so it makes some noise, otherwise
it won't sound much. So first you tap the string (on the 4th fret in this case),
then you bend the string a little bit (just a tad) and then you release it.
It might take a while to get it right, but it will sound really
cool and your friends will be impressed when you nail "Thunderstruck"
with your left hand while drinking beer with your right hand.
Good luck!
Edit: Led Zep, download it, It's a great song!
"Thunderstruck" by AC/DC