Posts are contents that you can create, edit, and organize listed on your website in a reverse chronological order.
Posts can be anything that you plan to publish on your website. For example, if you plan to create a blog, posts can be blog posts. If you plan to create a news site and doesn’t want to use a custom post type, posts can also be used for that.
As a general rule, consider posts for the content that your website will be mostly consisting of or for blog articles. Do not use posts for things like displaying your Privacy Policy, or Terms and Conditions. WordPress have Pages for that.
The same applies to other content types that require different functionalities that a post cannot provide (like a product page). A custom post type or a separate plugin like WooCommerce is better in that case.