Siloing in SEO means grouping related pages together, either through internal linking or structurally, to establish the site's keyword-based themes.
It is important because a silo structure helps search engines better understand a website and all its content. That is why small niche sites with properly built silos can sometimes even outrank authority websites.
Now, to create a silo structure, you can start with a top silo page that will link down to child pages below it. The idea is to create closed ecosystems — groups of tightly related content.
Each silo (or group/category) can have between 10 to 100 posts each.
While internal linking also, you can link posts in one silo (or category) only to build topical relevance. This includes all internal links: in-content, sidebar, and footer links.
The basic site URL structure with silos in place looks like this: homepage > silo page > blog post.