For further questions, please post on the appropriate subforum, aka "Beginner's help and Guides". And if you don't find your understand in all sorts of Guides, then you may start a topic.
To answer you, only "main" can take part in strategus battles. That is the "role" of one of your characters, while all others (if any) are "alts".
With a main, you can view the future battles, and apply for one side (or both). One of the two sides is ruled by players while the other can either be a neutral fief (village, castle, town) or another clan ruled by players. In the case of applying for other players, they know your level, and can decide wether to hire you or not. These days, it's pretty hard to be hired if you're new, clanless, without reputation, etc...
If you're applying for the neutral fief, then every hour, they'll recruit people, based on their stats (level + showed up when accepted in strategus battles + k/d in strategus battles). You need to be active in cRPG the hour before the battle, else you sometimes get kicked off the roster.
Strategus in itself is rather complicated. It's like singleplayer, but with players travelling all across the land, forming clans, raiding, taking villages and trading. You can find more information on the guides. Strategus is an awesome feature, but requires to be kind of active, and is, at the moment, a bit boring.
