The Randomness condition allows you to send out posts in your Recurring Posts List in a random fashion. The Randomness number represents the random factor (percent) for selecting the next post to send out from the end of the list of posts.
For example, with it set at 25 percent - if you had 100 posts in the list, the next post would be selected from the last 25 posts in the list. (The last posts meaning the posts sent the longest time ago.)
If you set the Randomness to 100 percent, then the next post would be randomly selected from all posts in the list.
Needless to say, any other condition you set up in your Recurring Posts List will be applied when selecting the next post.
If you remove the Randomness condition, then the next sent post will be the last one in the list (the post sent out the longest time ago).