Layout Builder is a core module that lets you arrange content visually on a per entity basis. It gives editors a drag and drop interface for placing blocks, fields, and custom content into layouts.
You can use it in two modes. Default mode sets a layout for all content of a given bundle, like making every article use a two column layout. Override mode lets individual pieces of content have their own unique layouts.
Layout Builder replaces older approaches like Panels and Panelizer for most use cases. It is well integrated with core, respects cache tags, and handles permissions properly. Some teams love it for giving editors real power, while others prefer stricter templates for consistency. Either way, it is now the default answer for flexible page building in Drupal.