Each rank you create has a minimum and a maximum balance requirement. As users gain or lose points on your website, their balance will fit in different ranks and so the user either promoted to a higher rank or demoted to a lower rank.
Ranks can be based on a users current balance or their total balance which controls if a user can be demoted from a rank or not. As an example, you might want your users to always keep their highest rank even if they spend / lose their points, in which case ranks need to be based on total balance instead of current balance.
Based on Current Balance
As the users balance increases, so does their rank. If the user spends or looses points, they also risk of losing their rank, based on your setup. Basing ranks on users current balance is best suited when your users only gain points.
Based on Total Balance
Basing ranks on users total balance allows them to retain their rank even if they spend their points. When using the total balance, the only way a user can lose their rank if an administrator deducts points from the user manually with a log entry.
myCred Ranks are a custom post type in WordPress and are managed just like you manage Posts/Pages or any other Custom Post Type. As a bare minimum each rank has a title, however in your add-on settings you can select to enable usage of other post related details such as the post content, excerpt or even comment.