A link pyramid is a structured form of link scheme using several tiers of links.
Link pyramids aim at creating link juice for intermediary and first tier links. It is also a way to prevent sanctions for the money site which is only directly connected with "good quality" first tier backlinks.
Link pyramid is way to represent tiered link building.
Packaged link pyramids are seldomly effective and may be dangerous for the money site because the pyramid may be detected by Google and because the first tier backlinks are often poor links.
Some examples of link pyramids:
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image credit SEO Doctor Z