The world tour is a friendly match tournament where you travel around the world to play clubs/managers at the same level as you.
Your players will earn routine when you play world tour matches, so it can be a good way of giving some match time for your younger players to build their routine.
The world tour starts once per month. If you have not completed your current world tour when the new one starts, don' worry - you will get to play your current world tour to the end and the new one will be queued and ready to start when you are done with your current one. You can at most have 1 world tour in queue though.