In order to make time for writing I must give up working or gaming...
"Rage" is created with cards. You take a card from your DRAW pile and place it face down onto an open action space. This counts as one rage. You can do this for as many open actions you have. You're sacrificing the cards from your draw pile since those are discarded upon refresh.
Rage is usually used so that the Soldier can keep his Threat nice and high (because unused Action Slots lower your Rage during the Hero Deck Phase). The reason the Soldier might want to do this is because he has lots of Interrupt Cards that are very good for protecting himself and attacking enemies during Darkness Cycles, so if the enemies gravitate towards him, this is best.
Not all monsters are attracted to Threat, however. Crawlers prefer Proximity and Undead ignore it altogether.
Cards like Battle Rage also convert Rage into a bonus.