While we're on the not-a-vampire trend we may as well do bleed, which also sounds vampiric but is not.
Bleed is what allows pictures to go right to the edge of the page - the picture 'bleeds' off the page, continuing into the paper that is trimmed off when making the physical pages. If the picture were positioned so that it went just up to the edge of the page and no further, it would rarely come out quite right. It requires too much precision, and if the page trimming were out by even a fraction of a millimetre, you'd see - and be irritated by - a tiny sliver of white paper at the outside edge of the picture.
The area around the actual page (trim area) into which the picture bleeds is the bleed margin. The bleed margin is typically about 3-5mm each side. When you see your page proofs, they will have crop marks in the corners. The pages will be cut to these crop marks. If you draw a line between the crop marks, you will see that some of the colour printing (if there is any) may fall outside the lines you've drawn. This excluded border is the bleed margin.
This really only matters to you if you are an illustrator. If you write only novels that have no illustrations, you'll only be able to spot the bleed margin on the proof of the cover and none of this will have mattered very much to you... But it's useful to know the term so that you don't look like an unprofessional ignoramus if your publisher mentions it.
If you're an illustrator, and the illustrations will bleed at one more edges of the page, you need to make sure everything important happens in the part of the picture that will be used - so not in the bleed margins. Only background should fall into the bleed margin, not a character's foot, or the end of their cape, or the monster's tentacles. Tentacles bleed if cut off, remember.