What does this mean, is it an hammer-on? Is it a pull-off? What is it?
The other answers already have good explanations of what this is. I would like to add that in order to understand this and other things you will encounter in the future you need more than someone telling you what to do in this particular instance. You need to learn to understand and read rhythmic notation. Tab notation sometimes includes rhythms, which are the horizontal and vertical lines above the staff. Read this page in order to get started with a basic understanding of how rhythms are notated and counted, it will be helpful to you:
Aside from Tab you will get better knowledge and understanding of music in the long run if you also start learning standard notation as well. Tab is great but it has its limitations. First, the Tab is showing only one way something can be played. On a guitar there are usually 2 or 3 options. Second, when you learn with Tab you find out where to put your fingers but really don’t get any indication of what you’re doing (what key you’re in, the relationship between notes, etc.).
It can't be either. Hammer-ons move to a higher and pull-offs move to a lower note - generally on the same string. That note stays on the D (2nd string, 3rd fret) for the duration of one whole crotchet (beat, here), and is written as two quavers tied across the centre of the bar. Writing like that makes it easier to read, and the effect is pushing that note early - just as the C does at the bar's end.
It's good that some tab actually uses standard (?) notation as well as fret/string numbers. At least that way, one is able to keep the timing as written.