If you're going to be away and need someone to watch over your account you can utilize the account sitting system to let someone log in to your account and take care of it. To do this, you need to click on settings on the top menu and then account sitting. From here you can enter the name of the player you wish to sit you. As soon as they accept the request, you will no longer be able to play your account until you end the account sitting.

You should definitely make sure the person sitting on the account is someone you trust. Since they have nearly full access to your account there is a potential for them to do a lot of damage. You also need to make sure they have time to adequately look after your account. To end the account sitting, just log in as normal to your account and you will be presented with a screen giving you the option to end the sitting.

When you've been asked to account sit for someone else, you will first need to accept the sitting request (or deny it, if you don't want to). You can do this by going to Settings on the top menu, then clicking on account sitting. Once you've accepted the request, you will see a log-in link next to the player's name. This will let you gain access to their account. 

Note: After a pre-set time restrictions are put on sat accounts, such as the inability to send attacks, supports, or trades. More information can be found on the settings page for each world and the announcement page for each world.