Ranks are titles and images which can be applied to a member upon reaching a set number of content items on your site. These will show up withi the posts of a member. You can set these up from the following area within your admin CP:
Once you are in this area, you will see the default set which will look something similar to the below. You can edit any of these with the pencil icon, or alternatively select "Add new rank"
On clicking add/edit you will see a screen similar to the below image, where you can add a number os 'pips' upon reaching a certain content count, along with a title for those members
As an alternative to using pips, you can add your own image to be shown upon reaching a certain rank by selecting the "Use a custom image" option. This will then give you the ability to upload an image of your choice.
You will note that at the top of the main list, you can select "change" on whether or not a user can change their own custom title. When selected you can set the number of content items a ember must have in order to change this from there profile.