The Randomize button is a feature that allows you to shuffle the images in your content bucket.  

Step 1: Go to your Content Dashboard on the left sidebar. 

 

Step 2: Select Your Buckets.

 

Step 3: Select the Bucket you wish to randomize by clicking on the drop-down button to the left that says Action and choose Edit.  

 

Step 4: Click on the randomize Bucket. 

Step 6: A pop up will show asking if you are sure you want to randomize.  Click Ok. 

The example below is what a bucket may look like before randomization.  You will notice several posts in a series that all look very similar.  If we were to schedule the posts as is, they would all show up in a row.  
Below is what the same Content Bucket looks like after we have Randomized it.  All of the posts have been shuffled.