basically, you preform your obedience daily.
when you do, you get various bonuses based on your level. most base obediences are +1-4 bonus to something specific, like poison or a skill. some are much better, like Geryon from the AP.
typically, 3rd level you gain your obedience, since 3 skill ranks are needed
typically, 12th level you gain the 1st boon. which is a SLA between 1-3/day
typically, 16th level you gain the 2nd boon. these vary A LOT between awesome and meh.
typically, 20th level you gain the 3rd boon. these can be super powerful or kind of meh.
PrCs can drastically reduce the level you gain these, to ~8, ~11, and ~14. doing so drastically increases the power curve that you character is when you get said abilities.
here's some crazy ones i mentioned over on Fun Finds
Nocticula let's you suck-face-ubus energy drain people or backstab for d10s
Mesmara can make you a ghost or get a grand hex
Worm guy gets you worm that walks template
Zura gives you a hideable vampirism
Sifkesh gives makes you a dismembered angel w 15ft reach
Andirifkhu turns you into a sandbox trap
Jezelda gives you true lycanthropy
Kabriri turns you into a ghoul
Mazmezz turns you into a drider
Gargoyle guy gives you a death gaze
And those are just off the top of my head.
If you do the Sifkesh boon cheese, you can get a handful of various boons that jump your sneak attack up ~7d6, not to mention A stacking flat +2/die based on the weapon. Toss that on a rogue 20 for ~200 SA damage/attack (Nocticula gives d10 SA vs. allies, just use a great disguise)
Alternately, you could use Sifkesh tricks to get your Cha mod to AC ~3-4 times