Theme sections are customizable blocks of content that determine the layout and appearance of different pages on your online store. You can add, remove, edit, and customize sections using the theme editor.
Shopify offers a detailed breakdown and introduction to sections
In your theme you have Static sections and Dynamic sections.
Static sections are visible in your store but their position can not be changed. This would apply to your Header, Footer and Mobile navigation as they will always appear in the same place across your store, regardless of their content.
Dynamic sections can be added, removed and re-ordered in your store to change the arrangement and content.
Sections contain their own settings that will apply to that section and Blocks of content that belong to a section.
For example a Slideshow will be a section, each slide within that slideshow would be a block where you are able to add images, text and other options.
Each of our themes include a wide selection that has been pre-designed and coded for an optimized display of content that you add to it. See our individual theme docs for a full list of sections included.