When selecting a format, what does the term bleed mean?
Paper formats with bleed are slightly bigger than the final size so that the edges of the paper sheet can be trimmed. After trimming, the final format is obtained. This is useful if the document has photos, images or colour areas that reach to the edge of the paper. Trimming the edges prevents unprinted strips where the original paper would shine through.
Have a look at the Wikipedia article on Bleed (printing), too.