Minmaxing is a term used widely in various circumstances now, lets google a definition and work it from there:
taken from dictionary.com
(in a video game or role-playing game) to optimize (a character) by assigning all, or nearly all, skill points to the ability essential to that character’s success in a specified role and environment, and no points to other skills, rather than distributing skill points more evenly across attributes.
While this definition is quite specific in scope of a role-playing game, it is usually meant in a general meaning of playing effectively. When you are a warrior, get as much strength as possible. When playing a magical character, get as much magical power and mana as you can. In scope of a strategy game you would focus on the important yields only and get the most value out of your units.
If we are playing Civ6 "minmaxing" I imagine these scenarios:
1. Science victory. Build as many campuses as possible. If boxed in, capture several cities, but do not conquer the continent. Build a few high production cities, chop all forests, build mines. Ignore religion and other great people, sell all relics and Great Works for immediate profit.
2. Domination campaign. Build several critical units, three siege units, great general and move them effectively on a conquest spree. Get some science and gold back home, but the focus is on your main army exclusively - manage promotion heals, farm pillages, movement points, great general coverage, unit support and flanking bonuses, etc.
3. Maximizing city yields. Plan and optimally execute building improvements, city districts, wonders, in order to get as yield-heavy city as you can.
It does not always have to be the most effective way to play the game, but usually is. As opposed to playing casually or "for fun", or just playing what the game gives you and seems okay at the moment.