Does this mean I have to turn the hob up to high first in order to heat the oil then reduce to moderate to fry the food?

That's how I'd read it.
What will you be cooking?

Tuna.

Also if the dial on my hob goes from 1-9, what number would be moderate?

Well, that should be 5.
1-3 low, 4-6, medium, 7-9 high.
If you're unsure try this
http://www.jamieoliver.com/videos/ho...gypoDkfdeiu.97

Ooh no, I like my fish cooked all the way through

Presumably you've aleady had your tuna by now, but for future reference, that's not necessarily what it means. It's literally just telling you to heat the oil until it's hot. That doesn't mean it has to be at high heat. It'll heat up just as well at the same moderate heat you'll cook at; it'll just take longer.

I cook tuna similar to a steak, oil the tuna rather than the pan, then get the griddle pan really really hot. The tuna will only need 30 seconds or so on each side.