I believe he means "ADCs that get a lot or most of their damage through spells".
(I reiterate: I am not a good ADC, and probably don't know what I'm talking abotu)
For example, ADC's like Vayne, Kog'Maw, and Jinx get most of their damage from their autoattacks, and their spells are either used for utility, or make those autoattacks better (Vayne's Silver Bolt passive, and Kog'Maws Bioarcane Barrage; they ARE spells, but they just enhance your Auto-attacks, rather than do anything by themselves).
A 'spellslinger' ADC, as TehNACHO put it (I believe the term "Caster ADC" is a more common term), would be like Ezreal, who gets a lot of his damage from his Mystic Shot; Lucian, who uses his Q (Piercing Light) and passive (Lightslinger) for a lot of their damage, is another example. They both auto-attack less than an Auto-attack ADC, and use spells for most of their damage.
I hope any of that made sense.
TL;DR: 'Caster' (Spellslinger) ADC's use Spells for most of their damage, with auto-attacks weaved between spell-casts (Ezreal, Lucian, Corki, and probably more, but those three are the best examples that come to mind).
Auto-Attack ADCs get most of their damage from right-clicking on the target, but may have spells that enhance or make those Auto-attacks better. They may ALSO have spells that can be used for damage, but they still do more damage over time with autoattacks than with their spells. (Kog'Maw, Vayne, Jinx, Caitlyn).
and is much better in a solo lane then as an ADC.
could work on 
instead of 
if you get huge gold influx before you finish it
