Breadcrumbs are a useful tool for letting your users know where in your site they are.  At the top of each page, you present your users a series of links allowing them to go back up to a parent section.

As an example, let’s say you have a food blog.  One of the categories of that blog is French recipes.  You then have a sub-category named Southern French and in there the recipe for an amazing cassoulet, a typical southern French casserole.  When your users open the Cassoulet recipe, at the top of the post the will see something like the following.

French / Southern French / Cassoulet

Your users can quickly see your Southern French recipes as well as the highest category of French recipes.

The benefit of this feature is that it gives your users a sense of comfort and familiarity as they always know where they are.