A hub site is a website that links to trusted authority websites within a particular market. Hub sites are built on outgoing links.
Hubs can also be individual pages. For example, a hub page for the SEO industry would link externally to high-authority sites within the SEO and digital marketing niche and would not tend to link to any low-quality websites.
Recent studies into search engine behaviour when it comes to external linking have shown that they place great value on high-quality external linking structures when considering a webpages potential to rank. While a page with a million links to bad sites will not achieve very much, a focused, carefully structured, and useful approach to linking has been shown to propel sites up the rankings. Particularly, good hub pages and sites are rewarded for linking out.
If you're considering a hub page for your site, keep the links per-page to a reasonable number to please the crawlers – Google recommend you don't go overboard. To be safe, stick to 100 - 150 links maximum.
Hub pages can also be used internally for sites with a lot of content to group content by topic. For example, a car sales site might have a hub page for each different car brand with internal links pointing to all the specific models from that brand.