If you play tactics type RPG, then you should sort of know how it works, if not, then it is tougher. At the start of a stage you get to
put whichever units you want in battle. You take your turn moving your units and attacking. Then the enemy takes it's turn. P attacks
mean it is a Post Movement attack, meaning you can attack after you move. The [ + ] looking thing means it is a danmaku attack, so it
uses the Range stat. The Fist looking thing means it is a melee attack, so it uses the Melee stat.
You can use special skills before the character completes their action. So, you can heal up everyone with that one character, given
that you enough SP. Between levels, you can equip characters, power them up by giving them bonuses or abilities and stuff like that.
Focus makes it so that you do not get penalties in the danmaku fields that the enemies create. Focus penalizes by making it so that
you move slower and you will still take damage if you dodge an attack, they call this grazing instead of completely dodging the attack.
Also, check the translated manual that comes with the translation patch to see more information.