It is putting on a thin layer of frosting that will seal in all of the crumbs a cake makes when you put on the frosting....First just do the thin coat then wait till it crusts or put it in the fridge for a little bit...then you can go back with your thick layer of frosting without the hastle of being to careful getting crumbs in the top layer of frosting! 
A crumbcoat is a very thin layer of buttercream put on to seal in your crumbs, so you wont get it in you icing when you frost your cake. It can also be apricot glaze or something like that. Hope this helps!
Apricot glaze sounds great to use for crumb coat. Coundn't find a recipe for it here. Can you send me one? [email protected] I've only used thinned out buttercream frosting to crumb coat before. Which brings up another question. Should I even thin out the icing first? I thought my instructor recommended it, but it's been a while since my class and I could be mistaken.
You get apricot preserves, Heat it up a bit and then strain it through a sieve. Voila!! You need a lot. Some people even use piping jell. Ugh.
Thank you fearlessbaker. I'll be trying it this weekend. I'll assume that you can use any flavor preserves. Will I need more that one jar for a two layer cake?
I read on the piping gel container (Wilton) that you can use it as a crumb coat. Willing to try anything, I did it. Works great. I've used it on the last three cakes I've done and haven't had a single crumb. In fact I used it last night on a chocolate (yes, I said chocolate) cake and iced it this morning with white buttercream - NOT A SINGLE CRUMB!! I put the piping gel on and let the cake sit overnight then ice the next morning/day and it works great! There is no taste difference in the cake and it makes it even moister! The icing goes on so much easier and I feel that I can smooth the cake even better.
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