If you look at the properties for your DocumentDB Account, you will notice that there are Primary and Secondary Keys for both read/ write  and read only 

You can only access DocumentDB with one key. Why do you need a Secondary Key?

Let’s say you want to change your primary key.

TIP: It’s a best practice to change your primary key when you are deploying from one environment to another.  Especially when there is more than developer using it.

With only one key, you would have to take your database off-line.  Having two keys allow you to update your configuration files to use the Secondary Key.  You can then regenerate your Primary Key and update your configuration files to  use it.

Using the read/ write  Keys.

If you are developing an API or MVC Application, it’s a good practice to generate a DocumentClient for both read/ write  and read only clients.

Queries in DocumentDB are always read only

You can read more about the use of  these keys in an article written by Ryan CrawCour  and another about  Account Management  written by Andrew Hoh.