Brownian Motion Brownian motion is the random walk taken by a particle which is constantly being buffeted by random collisions with neighboring particles. Einstein and Langevin first realized that the visible Brownian motion of dust particles must be driven by thermal collisions with invisible fluid molecules. This provided some of the first direct evidence for the existence of atoms. The magnetic vortices' random walk is not driven by thermal collisions but rather by their motion over the disordered energy landscape of the superconductor. An energy well is a region where the vortex has a low energy relative to surrounding regions.
Extra energy is needed to remove a vortex from such a region.
Pinning sites are energy wells. In a triangular vortex lattice, the vortices are arranged such that the vortices form equilateral triangles with each of their six closest neighbors, as can be seen by the black triangle in the image. The Boltzmann-like form of the distribution of the pinning energies means that the probability of a vortex having energy E falls exponentially as E increases. The protrusions aren't really sticking out. They represent the vortices, and here they are covered with yellow dots so they show up more clearly.