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It's not a dumb question at all! Please don't worry!
"Crested" means the kimono has crests on it. A japanese crest is called mon, and, as in western heraldics, represents a family, group, etc. A mon that specifically stands for a family (and not an organization/store/other kind of group) is called kamon. So a crested kimono features little symbols of the wearer's family, kamon, and therefore, is more formal.
The kamon in a crested kimono can be 1, 3 or 5. They are always put on the same position:
- 1 kamon (quite formal): under the eri (collar) at the back.
- 3 kamon (formal): same as 1, plus both sleeves (over the elbow, more to the back).
- 5 kamon (super formal): same as 3, plus both front sides of the eri.
Super formal kurotomesode features all 5 kamon. Less formal kimono can feature no kamon, just the one at the
back of the eri or that plus the ones at the sleeves. Beautifu kurotomesode found on K-Bridal.
Some kinds of kimono can have their formality "pushed up" by adding kamon (see the articles on kimono types for more details on each case). Other, like yukata, are too informal and never feature kamon. Why? Think that it's like wearing stilettos and a minaudière (you know, those tiny sparkly evening purses) with your gym sweater and shorts. It would just look unfitting together. ;p
I hope this answers your doubt, Raxiell! Thank you for your question! ^^