Vertical progression is the WoW model where there is only a singular endgame content where the best gear in the game is acquired from. Horizontal progression is the oldschool FFXI model where there is multiple endgame content in which you can acquire the best gear in the game.

For example: WoW only has raiding content for the endgame players and oldschool FFXI had: Dynamis, Sky, Sea, Limbus, Einherjar, Salvage and HNMs.

I should've said horizontal endgame content since some of these WoW clones out there have a sort of horizontal progression but it is worthless as the content this stuff is tied to offers absolutely zero challenge and is usually obtained via loleasydungeons or loleasy raid finder for noobs.