Basically how the game works is this:
There are town players and mafia players. Mafia knows who their teammates are, but town doesn't know who their teammates are. They are informed, but the minority, town is uninformed, but the majority. The goal of the game for town is to find and kill all the mafia players, while mafia's goal is to live long enough that they compose half of the remaining players.
Each "day" the entire group decides on someone to kill. It is majority rules, so in the first day, since there are nine people, five need to vote to kill a player. We "lynch" that player. Each "night" the mafia decides on one player to kill. It could be someone that suspects they are mafia, someone they simply know is a good player, or someone they don't want around for whatever other reason. The cycle then repeats itself. Day, night, day, night, until one of the two win conditions (mafia are all killed or half of the remaining players are mafia) is fulfilled.
Now there are certain "roles" that each player has. Some examples might be the cop, who can each night check whether a person is innocent or guilty. A doctor can pick someone to protect from being killed at night. In the last game you were a vigilante, so each night you could shoot someone you thought was mafia. There are also sometimes limits, which is why in the last game you were described as 1x Vigilante. That means you had one shot to kill someone instead of doing it every night. Most players are usually going to be vanilla, which means that they don't have any special powers and simply have the ability to think about who is behaving funny and vote for them.
So how do you go about playing? Well, if you write posts people can make informed guesses as to your alignment. For example, based on this post you might think "Well warren42 is trying to help out a new player. That seems genuine and like he's trying to help this player learn and win the game. That seems like something a town person would do." You might also think "Hm... Warren42 is trying to help out a new player and get that new player to think he's town, which would help him if he's mafia. He's probably mafia." Finally, you might look at this post and think "Warren42 is trying to help a new player understand the game. This post is not indicative of if he's town or mafia, as he could make a post helping out a new player either way."
If you're mafia, your goal is simply to have at least one of your teammates survive long enough. You want to avoid being suspicious. You also know who is town and who isn't, so you have to pretend you don't and that you're actually trying to determine if a player is town or mafia (like the thought process above.) Let's say you're mafia and Ragnar is town. You want to pretend to be trying to determine if Ragnar is town or mafia by faking a thought process on his posts and behavior.
Essentially mafia try to lie their way through the game while town try to spot the liars.