The main power sources of a space shuttle (and many space-faring vessels before it, such as the Apollo spacecraft) are the <a href="https://en.wikipedia.org/wiki/en:Fuel_cell" class="extiw" title="w:en:Fuel cell">fuel cells</a> (F/Cs), which combine hydrogen and oxygen into water and derive electricity from this process.  Most of the water is used internally in the spacecraft (either as coolant or used by the astronauts; it's perfectly clean molecular dihydrogen monoxide), but to avoid degradation of the efficiency of the fuel cells, excess water is purged from the cells into space through this port and another like it on the other side of the shuttle.
This photo of the Space Shuttle <a href="https://en.wikipedia.org/wiki/en:Space_Shuttle_Endeavour" class="extiw" title="w:en:Space Shuttle Endeavour"><i>Endeavour</i></a> was taken at the <a rel="nofollow" class="external text" href="http://www.californiasciencecenter.org/">California Science Center</a>, the "retirement home" (if you will) of this particular shuttle.  If you live in the area, go check it out; entrance is only $2!
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/25649741@N04">Andrew "FastLizard4" Adams</a> from United States
CC BY-SA 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/fastlizard4/8656540103/">Space Shuttle Endeavour Fuel Cell Purge Port</a>