You can allow external apps access to your TimeLog data. You log in with your user name and password, which provides the app access to TimeLog as you e.g. know it from Facebook and LinkedIn. You decide yourself, if you would like to allow access and to which data and how long.
When you log in, a token is given and it is valid for one hour, we can call it a single ticket for the roller coaster at a theme park. If you mark the Remember my login field, you are given a re-fresh token which corresponds to a season pass for the theme park, as it is typically valid for one year. If you select this option, you do not have to log in again all the time.