So, what's an Aegir?
Aegir is a tool to manage drupal websites.
Simply put: Aegir makes your life easier.
From the official website:
Aegir allows you to deploy and manage many Drupal sites, and can scale across multiple server clusters. Aegir makes it easy to install, upgrade, and backup an entire network of Drupal sites.
If your job is to manage content, Aegir lets you concentrate on that rather than complex and tedious computer stuff.
If you're a hosting provider, Aegir offers you stable and powerful mass management tools for Drupal.