Free experience in World of Tanks can be obtained in two different ways. You earn 5% of your regular experience income in every battle you play. You can also convert regular experience from elite tanks to free experience using gold.
Understood! But what is the free experience good for?
Free experience is automatically used whenever you research a vehicle you don‘t have enough regular experience for (as long as it is the next vehicle on the tech tree branch). Your free experience is valid for researching literally anything researchable in the game. Provided of course that you will follow the tech branches as they are growing. You cannot skip a few tanks in the branch and research only the tier 8 tank for example, you have to have all the predecessor tanks researched first.
How do I convert regular experience to free experience?
The process is very simple and you can find out how to do it here.
Check the costs first!
Be aware that in some situations, you need to carefully check which experience you are using or you might unintentionally use the free experience on something you don’t want to!
Before you research a vehicle which has more than one predecessor, please check this article.