Multisites allow you run multiple websites from just one WordPress installation. Each website can have different users, different themes and different plugins.
This is possible because the old WPMU (WordPress Multi-User) project was merged into the main WordPress system. This is the same codebase that allows WordPress.com to run millions of sites, so you can be sure it works well.
In order to set this up, you will need access to your files so that you can edit them. This tutorial will show you how. When you've finished, click here to read Part 2 of this tutorial: Managing WordPress Multisites