Strings that surprise: how a theory progressed

In August 1984 two physicists arrived at a formula that transformed our understanding of string theory, an achievement now recognised by a major award. Professor Michael Green of the Department of Applied Mathematics and Theoretical Physics explains how string theory has taken unexpected directions.
In December 2013 Professor Michael Green of Cambridge University and Professor John Schwarz of California Institute of Technology were awarded the 2014 Fundamental Physics Prize, one of a series of annual "Breakthrough Prizes" set up to raise the profile of the physical and biological sciences. The shared $3 mn prize was given for "opening new perspectives on quantum gravity and the unification of forces".

[@apekillssnake]

I cant understand the part about strings being "open-loops" and closed-loops. Is this a reference to open-strings being the Plancks length and closed-loops being stuck on a brane, or are both open and closed loops within both places of extra space?
Nor can I find any videos that go into this detail.