The -2 means you have two setters rotating (as opposed to strategically subbing) in. The 5-2 has one setter rotate through the back row and the other rotate through the front row, but only one setter on the court at a time.. 6-2 has the two setters, but only rotating through the back row, again only one setter at a time. The more conventional 5-1 uses just one setter that rotates through all six positions.
This refers to their starting positions (meaning until the serve) rather than where they actually set from. So it affects the serve-receive formation in addition to providing two setters' skill sets.
Sort of a corollary to all of this is the "overlap rule." In a given rotation, there are three front court players and three back court, and a right, middle, and left. At serve, each of the front court players must be in front of the corresponding back court player, the two left side players must be to the left of corresponding middle players and right to the right of corresponding middle players, even if pretty slightly. (It's hard to state this accurately in words, a lot of sites explain this with varying clarity).