What does garrisoning units in cities mean?
Garrison is just that,
garrison, it will defend whenever an enemy unit tries to move its units inside your city.
You can not have to much defense, however, units cost upkeep to maintain and shields to produce,. You may wonder whether the value of a units being at a certain location is worth the upkeep and production it costs. (
most of the time, units garrisoning a city are a waste)
I know it is to establish order after civil disorder
The way you say it appears to indicate a lack of understanding how civil disorder and happiness/contentness work.
A city will be in civil disorder the
next turn if it has more unhappy citizens than happy. (content citizens don't matter, specialist are always content)
So if a city has 6 happy and 6 unhappy, its safe! If it has 3 happy, 6 content and 3 unhappy, its safe! If it has 3 happy 5 content and 4 unhappy, then it will be in disorder, unless you can resolve it before hitting the
next turn button.
How you resolve unhappiness issues doesn't matter, except that one method is more efficient than an other, depending on the situation.
Under certain government types, up to a limit, units in a city provide 1 content face per unit. This is called
"MP" (Military police)
This is just one of many ways to resolve unhappiness issues though. You can also add contentness-effect with temples, cathedrals, colloseums, and some Great wonders. Or you can add happiness-effect with lux resources, or withe the lux slider on the F1 screen.
In addition, military units can also
squelch resistors, but this is an other thing entirely. Resistors are citizens in a city you just captured that don't do anything for you, prevent rush-building, and increase flip chance.
I don't know the exact formula, but leaving many units in a city will hasten their assimilation and acceptance of your rule. This has nothing to do with MP/