When Leanplum starts for the first time in your app, it creates a new user profile or User ID. Unless you set up your own concept of User IDs, the UserID value will be identical to the DeviceID value, and Leanplum will start tracking sessions for this user.
Sessions are how Leanplum organizes each user's events, states, and other general activity.
See below for more on handling user IDs, devices, and sessions.