A silo structure is a type of website architecture where content is organized logically. It consists of hierarchical groupings including topics and subtopics. In other words, you can think about it as creating categories and sub-categories for your site and writing relevant content for them.
Take this further and cover every query in every topic in an entire niche, and you win the internet. This is how silos work — they let you grab your main ideas and break them down into smaller and smaller categories until you have pages that are answering all relevant user queries.