4.1.1. Block layout

Structure -> Block layout
The blocks determine which content should be displayed where on the page. The settings for the blocks can be found under the menu item Structure -> Block layout. Certain contents are displayed per block, e.g. menus, content, views, etc. Some blocks are already preconfigured, e.g. "Main navigation" or "Main page content". 
Each block can be assigned to a "region", i.e. a page section. Which regions exist depends on the theme used. Under "Demonstrate block regions" you can display the existing regions.
Under Block layout you will find a list of all currently used blocks. Here you can move the blocks to other regions, deactivate them (Disable), activate them (Enable) and configure them (Configure).
Under "Configure" you can change the title of a block, show or hide it and limit the visibility to certain pages or user roles.
The contents of the block itself can be changed in the respective menu, content, view, etc.