This is a section of a webpage which helps users understand their location within the whole website. It is coded by the webmaster and usually appears close to the top of the page.
An example can be seen on this page where the breadcrumbs are:
Home > SEO Glossary > What are Breadcrumbs?
This states that the route to this page (from the home page) is via the SEO Glossary page.
Breadcrumbs are extremely helpful to internet users in very large websites and especially in shopping sites where they can confirm they are browsing in the right area. For example:
Home > Electronic Goods > Televisions > LCD
In the same way breadcrumbs help search engines structure your website correctly in their index. Recently Google added breadcrumbs in their search results and they can be seen in green text below the description if the website uses them.