I've commented the code for you, hopefully it makes sense.
Sub macro1() ' macro name, performs no action
For i = 2 To 100 Step 4 ' starts with i as value of 2
Range("A" & i) = ZipConvert(Range("A" & i)) ' copies value of ZipConvert!A2 to A2 on the visible sheet
Next i ' repeats the above process, adding the step value to i, so A2 will become A6, then A10, etc. will stop when i goes above the limit value of 100
End Sub ' confirms end of macro
To use it you need to go to the macro's section in the view menu, select macro1, then run.
I'd have to respectfully disagree with jasonb on the zipconvert line. My guess is that zipconvert is a Function (additional vba code) that converts a number like a zip code into some other format or something like that.
Good catch Alpha, don't know what made me see that as a worksheet name, probably doing to many worksheet formulae recently and forgotten what VBA syntax should look like, either that or it's just my brain telling me that on a saturday evening VBA should stand for Vodka Beer & Absinthe