what is brigades does means?

When you or someone attacking your country send units to make the attack those units make up a "brigade".

So you or the country you attacked get a message to say that a "brigade" invaded their land (or was beaten off). For most attacks the units sent, on completion of the attack, fall back into the "standing army" and those units are available to be sent on another mission.

But if units are sent on a Planned Strike (PS) they stay set aside as a brigade for a period of time and cannot be used again until they fall back into the standing army. If you click on "military" you will see details of the units in your standing army and of any brigades which are set aside because they have returned from a PS. You are also told how long it will be before each such brigade falls back into your standing army.

thx, so the main menu written 5 brigades available, does that means I can only do 5 plan strike?

Yes.

If you do 5 you can then do no attacks of any kind with your units until one of the brigades gets home.

On the Express server some people occasionally do five PS attacks (on that server the brigades get home again pretty quickly) but on other servers it is better only to do up to 4 so that, at need you can still make other attacks.

thx