Backlinks
Links from other websites pointing to a page — still one of the main signals search engines use to judge importance.
A link from another site works as a reference. Links from relevant, trusted sites carry more weight than many from unrelated or low-quality ones, and links bought or exchanged for ranking purposes break Google's spam policies.
Backlink counts in SEO tools are each tool's own estimate; no tool sees the whole web the way a search engine does.