What is Power Cosmic?
Presumably immeasurable power which was described as "the most powerful force in the universe." The primary wielder - and perhaps the primary source - of the Power Cosmic is the god-like cosmic entity Galactus.
The Power Cosmic allows Galactus to employ the available absorbed cosmic energies within him to produce nearly any effect he desires, including size-alteration, the molecular restructuring and transmutation of matter, the teleportation of objects—even entire galaxies—across space and time, the creation of force fields, the creation of interdimensional portals, telepathy, telekinesis, and cosmic awareness on a universal scale. Galactus has even been shown as capable of creating sentient life, resurrecting the dead, manipulating mortal souls and remaking dead worlds—including their populations—in every detail.
The Silver Surfer, as well as most of the Heralds of Galactus, possess certain measures of the Power Cosmic. Bestowed on them by Galactus during their service to him, their abilities and assigned elements of influence vary, but the power can usually be expelled violently as concussive force, and sometimes, among other things, used with precision as a means to restructure molecules according to the user's mental design. However, heralds are imbued with only a fraction of the amount possessed by Galactus, and the latter has repeatedly been shown able to strip them of the power he has afforded them.