As they noted in their paper, pages stuffed fulled of useless keywords “often wash out any results that a user is interested in.” While we often complain when we run into spammy pages today, the issue was far worse then. In their paper they state that, “as of November 1997, only one of the top four commercial search engines finds itself (returns its own search page in response to its name in the top ten results).” That’s incredibly difficult to imagine happening now. Imagine searching for the word “Google” in that search engine, and not have it pull up www.google.com in the first page of results. And yet, that’s how bad it was 20 years ago.
These are ‘tit-for-tat’ links. For instance, you make a deal with your friend who has a business website to have him place a link to your website, and in exchange your website links back to his. In the dark ages of SEO, this used to be somewhat effective. But these days, Google considers such 'link exchanges' to be link schemes, and you may get hit with a penalty if you're excessive and obvious about it. This isn't to say that swapping links is always bad, but if your only motive is SEO, then odds are that you shouldn't do it.
A breadcrumb is a row of internal links at the top or bottom of the page that allows visitors to quickly navigate back to a previous section or the root page. Many breadcrumbs have the most general page (usually the root page) as the first, leftmost link and list the more specific sections out to the right. We recommend using breadcrumb structured data markup28 when showing breadcrumbs.