Sentence:  The resolution of the images obtained by this American probe [Messenger] will depend on its altitude [above Mercury] at any one time: about ten meters at perigee (200km altitude), but only one 1 km at <WSD>apogee</WSD> (15000km). 
Meaning of the word marked with <WSD> tags:  The point, in an orbit about any planet, that is farthest from the planet: the apoapsis of any satellite.