>A satellite constellation is a group of artificial satellites working in concert. Such a constellation can be considered to be a number of satellites with coordinated ground coverage, operating together under shared control, synchronized so that they overlap well in coverage, the period in which a satellite or other spacecraft is visible above the local horizon. [1]

[1] https://en.wikipedia.org/wiki/Satellite_constellation

In addition to sibling comment, a real-life example is the GPS constellations. A single GPS satellite won't do you any good, but two or three (or better yet, six or twelve) will pinpoint your location. So the US DOD, et. al., threw a bunch of them up there at once instead of just one.

And perhaps not just the military. Iridium wants to replace their constellation of satellite phone satellites? Is next week okay?

Some additional information to augment the info provided by the other replies. One more recent example would be the RADARSAT Constellation Mission (RCM):

http://www.asc-csa.gc.ca/eng/satellites/radarsat/default.asp

Having a constellation of satellites basically allows for 24/7 coverage of any given surface of the Earth, given the spherical nature of the Earth.

Full disclosure: I worked on RCM (linked above)