I've seen those in many songs. For instance look at the chorus in Typical Situation. Or in the main verse in #41. If it wants u to play a string open, in a chord or something, sometimes it will have an "h2" or an "h4" next to that "0".
Random movie quote to make you seem hip and "with it": The food court is downstairs. The cookie stand is upstairs. It's not like we're talking quantum physics here.
It means you play a string open, then hammer on to whatever fret the number is (in this cas 2 and 4) Do you know how to do a hammer on? You play the first note (open in this case) let it ring for a second then bring your finger down hard on the second not to make it ring out again. Try playing the opening line of where are you going to get a good feel for it, thats a good example.
