PPE or P.P.E. is the common abbreviation for Potential Psychic Energy. Every living thing possesses PPE in varying amounts.
PPE is the fuel that makes magic spells possible. In the past, nobody contained enough PPE within a single person to invoke it into a spell, but with the coming of the Rifts, vast amounts of Psychic Energy flood the earth, making it a readily available resource for those who can control it.
For example, a normal adult human has 2D6 PPE while a human child have 5D6. For comparison, a Mystic could have between 40 and 100 or more.