Meditation crystals were naturally-occurring rocks that assisted in inducing meditation. Growing naturally on the Deep Core world, Tython, slivers of a meditation crystal could be placed inside the hilt of a lightsaber to produce a coherent beam of energy, as well as assist the wielder to stay in closer connection to the Force during combat. This enhanced connection allowed the user to retain energy while fighting rather than losing it.
Behind the scenes
In The Force Unleashed II, these crystals are used by Marek for the majority of his adventure, and allowed him to regain force energy every time he struck an opponent.
Despite Marek appearing with two of these crystals in cutscenes as early as Dagobah, it is impossible to obtain more than one in-game before reaching the Salvation—though, according to the novel, the crystals are only blue on Dagobah during his vision, and he still had red-bladed weapons at the time. He received the Meditation crystals on the way to the Salvation in the novel, while in-game he acquires the second crystal around halfway through the Salvation.