Composer is a dependency manager and package manager for PHP that allows projects to declare their dependencies in a composer.json file. It installs dependencies and manages autoloading so that dependencies are available to a project. The presentation discusses why Composer is useful for avoiding dependency issues, how to initialize a project with Composer, add and update dependencies, and how Composer can be used to manage modules and themes in Drupal projects. It also covers using Composer scripts and plugins as well as integrating Composer and Drush.