When you are AFK for a certain amount of time, the game will kick your character out of the server, so you need to log in again to keep playing. This is mostly not really a problem, but it can be annoying if there are queues on the server you are playing. With this little trick you can stay in the game for as long as you want, and the server will never kick you offline.
What you need to do is to find a bonefire or a fireplace that damages your character. Most of these provides 25 damage every few seconds. This is so low that you will always generate more health then you loose, so you can stand in this fire forever. However, when you are taking damage, the game will not kick you offline, so your character will stay logged in until you move away from the fire.