That means no offset at all.
Just FYI.... what they're talking about, is on serious-effort heads, the push rods are extremely in the way of growing the intake ports, and one way to gain clearance there is to use offset lifters (real rollers of course, not the stupid factory rollers, remember we're talking about serious effort here) and rockers with the push rod end moved outward, to allow a wider intake port. The same sort of thing happens at the other end of the rocker; in order to put larger valves than 2.02"/1.6" in a set of heads, the guides have to be moved apart in the casting to a greater spacing than stock, which then offsets the valve end of the rocker outward too. Of course, most people who would be building motors of this sort, are using shaft-mount rockers such as Jesel; not stock-style stud-mount ones ones. The S2 heads aren't near big enough to need any of those adaptations.
So this would fortunately fall into the class of things of "if you have to ask, you probably don't have to worry about it".
I'd suggest the cast steel-bodied roller rockers from Comp, either the 1300 series, or if you feel like really biting off, the 1100 series. If you're going to run these on the street, avoid aluminum; and of course, no matter what, avoid any kind of cheap stamped sheet-metal ones.
