The variables module you to create variables that are accessible throughout your PyroCMS code. They are a good way to set small bits of text that you or other admins may want to change later. For instance, you can create a variable for the site's official Twitter handle. If it ever changes, it can be easily changable via the variables module.
To create a new variable, go into the Admin Panel, and select Variables from the Data menu. Click + Add Variables and create a slug and a value for your variable. The slug is what you'll use to refer to the variable in your layouts.
Once variables are created, you can edit them inline by clicking Edit.
Using Variables in Templates
To see how to use variables in layouts, see the variables plugin docs.