Use Flat Catalog Category and Use Flat Catalog Product are two options you can enable or disable in the Magento backend under System>Configuration>Catalog>Frontend. It is recommended that you enable this feature and here is why.
All the product and category information is saved in the database. The database consists of a bunch of tables, like excel sheets. There are lots of tables, and even more rows in each table. So when Magento has to search multiple tables for information that’s spread out, it can bottleneck and become slow. So enabling Use Flat Catalog Category and Use Flat Catalog Product will take the information that’s spread out and flatten it into one table (or relatively less tables) usually reducing the response time.