Emblems are what you get when you donate to a shrine and gain 1000 favour points (there are other ways to earn favour but this is quickest and most obvious). Humbaba is one of the giants, and she has her shrine somewhere in the world (check the map).
Once you have donated enough to earn 1000 points, the shrine will give you an Emblem. Then you can see all the things the Emblem does for yourself :) But you should know that spending it makes it disappear from your inventory.
Usually, giants are appeased with higher ticket items. Although they will reward you for any donation, the more expensive the donation, the more reward it gives.
Also, you should find some hallowed shrine powder, or just make some! It'll momentarily raise the amount of favour you receive with a giant you're donating to. Try it!
About the shrine powder: does that need to be sprinkled before each donation? ie: *sprinkle* *donate moonstone* *sprinkle* *donate pleasing amber* (i think it does) anyone confirm?
If you look at the details of the "special status" conferred when you sprinkle the shrine powder (in the bar just below the map at upper right), you'll see it persists for "the next donation", so once the sprinkling is complete, the effect is activated by the one next donation. You do have to sprinkle before each donation for which you want to see multiplied benefits.