It is the coat you put on just before the fondant that makes the fondant stick to the cake. It is usually BC, but you can use cream cheese, ganache or other frosting/icing. It should be thin, transparent and as smooth as you can get it (ganache is a bit thicker).
I often use a hot knife to smooth mine. It also prevents imperfections in the cake from showing through the fondant and allows for a smooth finish on the fondant. It is called a crumb coat because it mixes in the crumbs with the icing.
HTH
some people use crumb coat .. before applying next coat of american buttercream or imbc, smbc,etc. apply first coat litely. let set up.. then apply final coat... of whatever buttercream you are using.. hth
By doing this it eliminates crumbs in your final coat. (I thought I would add this because I remember someone once ask how to keeps crumbs out of the icing when icing their cake.)
Duplicate post, please also see:
http://www.cakecentral.com/cake-decorating-ftopicp-6702639.html#6702639
HTH
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