Due to the different numbers of each rarity type in the card pool, the way this works out is that each individual card has an identical 5.263% chance to appear
Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities, such as mobilize (from the Tarkir: Dragonstorm release), are triggered abilities and will have "when," "whenever," or "at" in their reminder text
A good practice is to always evaluate your hand for the best Trainer card to play at the start of your Action Phase, maximizing its impact
There are even special enchantments called classes, which you can invest mana in to level them up and gain even better effects
These blessings would help your smaller creatures fight harder in the Angels absence, allow the Angels to play well around friendly sweepers and set up synergies with cards like Guardian of Faith