A permission is something that is essentially an on/off switch for your user accounts. There are multiple switches for each channel, and each switch turns a part of that channel on or off for a specific account. These will all be available by clicking the gear icon in the top right of the application and clicking "Account Information". This should transport you to the account management dashboard, where you can click on "Multi-User" and then on "Permissions" to see what permissions are assigned to the account. These are broken up by channel and are fairly straightforward.
Permissions for the Twitter channel allow you to:
By splitting permissions you can avoid jurisdictional issues and people accidentally altering data in incorrect places. You are able to organize, compartmentalize, and manage your projects without having to worry and check in on every project by scrolling through all the projects. Just check each user, and since they only have specific permissions, you can check their work specifically. Alternatively, you can have specialists for each channel, allowing them to focus on that channel without the distractions of other features.
Multi-user is a 360 feature, so only available with a corresponding package. If you aren't sure if you have one, check our marketing automation page, and you'll be able to compare your feature list to that. Any package that includes more than one user would most certainly include the permissions system.