On the quest, where you have to choose you mastery you get a big information screen what each mastery gives you (a passive bonus + 1 active skill)
Also in game when you are in the passive tree [P], you can hover over each mastery and see it’s passive bonus and when you click on non-mastered masteries you also see the passive bonus + the active skill it would grant.
Would you like to have a quick summary here in the forums? Could give you that too if you want.
While you can’t change the mastery, you are still able to invest point’s into the other not-mastered masteries and can reach up to 25 poitns investment in them(to unlock skills)
You can also put as many passive points into them as you want.