Its usually people talking about the need for violence against the government (or whoever is the hated person or group of people in the thread). Its called fedposting because it is to be assumed that anyone willing to talk about violence in the open like this is actually a fed trying to entrap people, and even if it isn't, its probably someone trying to attract the fed's attention to whatever group the fedposting is taking place in. As a result, a shit-ton of euphemisms have cropped up so people can talk about violence against the government without being accused of fedposting, like watering the tree of liberty, or doing something in Minecraft, or anything to do with boogaloo, or the boog. All of the mentioned euphemisms are now super obvious, so don't use them.

Fedposting usually make you unwelcome in whatever group you belong to, so fair warning: don't fedpost. Save that kinda talk for your IRL friends or fellow militia members, and for god's sake don't do it on an unsecured line of communication.