Distributions are full copies of Drupal including Drupal core along with additional software such as modules, libraries and installation profiles.

Distributions are used for:

Demoing Drupal
Evaluating Drupal
Quickly binding Site
Learning Drupal

In the context of Drupal, a distribution refers to a pre-configured package of Drupal core, contributed modules, themes, libraries, and configuration that together provide specific functionality out of the box for a particular use case or industry vertical.

Here’s a concise answer for an interview scenario:

“Drupal distributions are pre-packaged sets of Drupal core, themes, modules, and configurations tailored for specific use cases or industries. They provide a quick and easy way to set up Drupal websites with specific functionalities without starting from scratch. You would use a Drupal distribution when you need to build a website that aligns with the features and requirements provided by that distribution, saving time and effort in development and configuration.”

Expanding on this, you could also mention some examples of popular Drupal distributions like Open Atrium, Commerce Kickstart, and Acquia Lightning, and explain the benefits of using distributions, such as accelerated development, reduced configuration time, and adherence to best practices.