Session depth refers to the number of banners a user sees in a session. The first time you log onto an app in and see an ad is Session Depth 1 and each new ad a user sees counts another. If you leave the min and max session depth at 0, you exercise no preference.
Go2mobi is the only self-serve Mobile Ad Network that allows you to target your campaigns on session depth. You can do this by utilizing the Min & Max Session Depth tool found on your campaign setup page.