There are a variety of factors that webmasters and SEOs can take into account when determining whether or not a page should be indexed.
One of the most important factors is how well the page has been optimized for search engines. Another factor to consider is how valuable the content on the page is to users.
Pages with high-quality, relevant content are typically more likely to be indexed than pages with low-quality content.
Archive pages like Tags and Categories are often noindexed.
Basically, think about what you want to hide and add the tag if needed.