Ok, so I understand why it’s useful: vertex and tri reduction.
And I understand that I really need it!
But I have no idea (and not for lack of searching) how it works or how I can implement it into my unity game!
I could really do with someone explaining it to me, so any assistance would be greatly appreciated! Thank you!
Are you talking about optimizing meshes option in the settings? ie turning the triangle soup into triangle strips with shared vertices?
Ah. Possibly. 
All I’ve been told is that I need to utilise ‘greedy meshing’ but I don’t know how. So are you suggesting it’s actually a settings option?
yah look it up
Greedy meshing as a technique is heavy on verts, not more optimised. Look into Poly2Tri for an alternative.
It doesn’t sound as if you know what you’re talking about. Do you need this for any purpose or are you just going on random irrelevant information? If you want to speed your game up consider using Simplygon (from asset store, free account) or other LOD management schemes. These are typically done via programs and not in code (unless you’re doing voxel stuff manually).
Above all I would ignore all this until you have a game. It probably isn’t a problem. If it was then you’d already know what you needed to do much more specifically.