From drupal.org - Concept: Blocks:
Blocks are individual pieces of your site’s web page layout. They are placed inside the regions of your theme, and can be created, removed, and rearranged in the Block layout (admin/structure/block) administration page. Examples of blocks include the Who’s online listing, the main navigation menu, and the breadcrumb trail. The main page content is also a block.
Some modules make new blocks available for placement on your site. For example, when the core Search module is installed and configured, it provides a block that contains a search form. You may also create and place your own custom blocks.
Each block has its own configuration settings, which allow you to select which pages of your site will display the block. It is even possible to place multiple copies of a block, each with its own separate configuration and visibility rules.
Blocks can be found at Structure -> Block layout.
You can configure a specific block by clicking the Configure button next to that block.
You can configure a blocks visibility settings (i.e. where they will and won't show up), based on:
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Language (If Multilingual is enabled)
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Content Type
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Page
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User Role
You can either place a specific block into a region by configuring the block, or by clicking the "Place block" button for a particular region on the Block layout page.
