Valor is a reservoir of mystical energy that trained warriors build up within themselves, and which they use to power their Techniques. Every Technique you use expends Valor, unless you have access to an enchanted item or other effect that allows you to ignore the Valor cost of a Technique.
“Valor” as a form of mystical energy is just one name for it; the same energy is also called ki, stamina, heroism, and a number of other things depending on which culture or scholar you ask about it. Whatever it is called, it is a naturally-occurring energy, growing within someone who has devoted themselves to developing their body’s physical capabilities and martial skill. Once it reaches a sufficient level, warriors can begin training to use this force, expending it to enhance their blows with deadly levels of power or to recover from what would otherwise be mortal or crippling wounds.
The existence of Valor and the Techniques that it fuels throughout the mortal peoples and in many of the beasts and monsters of the world has created a significant leveling effect within societies and militaries across the Continent. While some might be less physically blessed by their blood and nature, it’s unwise to take any warrior lightly — it doesn’t matter how large or small they are, if they can use Valor to cripple of kill someone with a single strike of a sword regardless of their size.