To expand on Droll's post -- you will want to ensure you either have
1) More "Police" strength (generated by ground forces and their commanders) than the "Resistance" rating
or
2) More "Protection Actual" (generated by military ships with weapons) than the "Required" rating
Only one of the two above need to be true for the unrest to go back to normal.
Both Police/Resistance and Protection/Actual are visible for each colony in the Economics Summary, in the middle of the 3 panels. The values are specific to each colony. While your home planet will still display these numbers, unrest will not actually increase on your home planet if it lacks has enough protection/police.
Police Strength:
In order to get more police strength, its the numbers of "units" within a formation that matters. This means, Light Infantry with Light weapons give you the most value for the industrial cost when it comes to having enough police. But that has to be managed with the knowledge that at some point, a planet could be invaded, and 10,000 police with tasers aren't going to be standing up to space aliens. So there is a tradeoff between good policing, and being able to defend well. If you are interested in the actual math behind the police rating, it is outlined here
https://aurora2.pentarch.org/index.php?topic=11593.msg136342#msg136342Note: The occupation bonus of the commanders really makes a difference, my belief is that in the current version (2.5.1) it is counted twice. So if you are manually assigning your ground unit commanders, that's something to prioritize on a situation like you experience.
Protection Strength:
I'm less familiar with the math behind this, but my understanding is that the driving force is more guns on the ships in orbit = more Protection. This means smaller, cheaper weapons like Plasma Carronades are actually really good at
oppress making your colonists feel safe. But again, tradeoffs, because you may not want short-range space shotguns taking on alien missile fleets.