Breadcrumbs are, uh, pieces of bread. Often generated by ripping bread apart with bare hands, by cutting bread with knife that's not sharp enough, or by eating the bread with no manners. They can be left on places to mark paths in maze-like places (small stones are usually better for this, see the following point) and they can be fed to birds in parks (where not forbidden). Most people prefer to eat breads whole, but I suppose breadcrumbs go well with some other kinds of food. (Milk or hot chocolate. Mmm.)
In hypertext, "breadcrumbs" refers to the first use mentioned above. Breadcrumbs (in some way or another) show which pages the user has been in and how s/he got there. In many WWW sites, the breadcrumbs are often shown as hierarachical link set ("Home > Subdirectory > Another subdirecotory > Document" or something like that). Or, something showing the visited documents. Or, as in minimal case, the browser keeps track of the information - link colors are changed.