Power level
Power level is how the game rates a sigil, from -3 for Gem Dependant up to 5 for Trifurcated Strike, Omni Strike and a handful of others. Negative numbers flag drawbacks. Brittle sits at -2 and Annoying at -1, and those are the sigils you want to keep off your own cards.
Activated vs passive
Eight sigils are activated, meaning you click them and pay to fire them. Bonehorn spends 1 energy for 3 bones and Power Dice spends 1 energy to reroll its power between 1 and 6. Everything else fires when its condition is met. Act III adds six conduits, which power the cards placed between two of them.