Home page sections are how you change the layout of your store. If you think of all the sections of your home page as small independent modules that can be dragged up and down to change their positions. For example if you had a Featured Collection section and you decided you wanted that at the very top of your site you can simply drag the section and move it up to a new position right in the theme editor. Some sections such as the header and footer can't be moved.
Blocks within sections
Within each section there are smaller modules which can also be dragged into new positions within their section. For example if you had a video gallery section, you can then add videos to it as blocks and drag and drop the order you want the videos to be displayed. With most sections you can keep adding blocks until your site is laid out as you want it.
Learn more from Shopify: https://help.shopify.com/manual/using-themes/change-the-layout