Here (left) we have a pair of D-branes, with superstrings stretching between them.

When the brane-brane strings are stretched, their vinrations have masses proportional to their length times their tension, and hence they do not generate addtional fields.  If however the branes become coincident - that is, if they occupy the same place or position, new massless fields enter the theory. 

These new fields coming from brane-brane string modes with coordinates parallel to the spacetime world-volume produce extra vectors in the theory.   For N coincident branes ("N" means a certain numner, or any number), the gauge symmetry gets enhanced to U(N).  ("U" being the symmetry group.  The larger N is the more complex the group).  There are also N scalars coming from the modes with coordinates transverse to the world-volume. These are the positions of the N branes in the transverse space. 

Moving the branes away from each other will break the gauge symmetry from U(N) to a subgroup. This is the Higgs mechanism.  The motion of branes in space-time may be described by the simple dynamics of the gauge theories which arise on their space-time world volumes.

Information provided by: http://www.kheper.auz.com