On the main login screen two buttons appear after you enter your username and password: "Kick" and "Kick&Resign". But why do we need them? Under normal circumstances, you actually don't need these buttons.
1) "Kick" removes your session from the server. This is very useful when you try to log in but you get an "Already logged in" error message. It's possible that you actually are still logged in, in another tab or on another device. "Kick" will do a remote logout on that other session. Unfortunately, it's also possible the server got confused. It sometimes happens when your internet connection has a strange hickup or your computer goes to sleep mode. The server never noticed you leaving, and this button explicitly instructs to clean up.
Note that while Kick removes your session, it does not remove your game. If you were in the middle of a game on that other device, you can actually log in after kick and you will be reconnected to your old game. However, sometimes you won't want that. If your game is broken/stuck, Kick will not save you. You will just get reconnected to this broken game over and over again.
2) "Kick&Resign" is a button that should only be used in a single case: your game is stuck/broken. If you face the circumstances in which you need to press this emergency button, please also report on forum about the issue you encountered. While you will be able to play a new game now, we'd still like to find the cause of the original problem.