As the cards say, Pokemon Lv X can be placed on the Pokemon it depicts and gives them a new move or Pokemon Power/Body to use. For example, Gliscor with Gliscor Lv X on it can use Night Slash in addition to any attacks the ordinary Gliscor had. The weakness, resistancee, retreat, and HP aso go to the Lv X.
Lv. X Pokemon are pretty much there with EX and Prime Pokemon in terms of Power.