RL (Real Life) is a designation for everything outside the game. It is commonly used in Tibia, but also in other games as well.
Example: In Tibia you might be a level 118 female Druid, but a twelve year old boy in Real Life. You can talk about RL in Tibia in both English Chat and World chat, or you could visit the RL board at Tibia.com Forums.
There are also numerous references to real life that can be found in Tibia. For more information, see the page on Allusions.
Sometimes, Real Life causes racism, a problem in Tibia that should not exist. It is better not to judge a Tibian on his Real life appearance, but only on his in-game behavior.