Free experience in World of Tanks is gained for each battle any player plays. 5% of the total experience a player earns each battle becomes free experience which can be used to research any tank/module and is not tied to any specific tank. There is also an option to convert the experience on elited(fully upgraded) tanks using gold into free experience to be used on other non-elited tanks. This can be a money pit for many who lack patience and want to skip up to the higher tiers quickly. While this might sound like a good idea it will burn through your gold very quickly and higher tier battles aren’t always as enjoyable as low tier battles.
Instead using free experience and converting experience should be used sparingly to skip past stock tanks. Instead of spending a ton of gold and free experience on skipping tanks completely it is more cost effective to only skip past the stock configuration. For example you can use your free experience to research the top gun/turret set ups to skip you past the horror of being an under gunned stock tank. This makes the grind much less frustrating and faster while not burning a hole into your pocket.