ConstituentPAC.com is a PAC. Here's how Wikipedia defines PAC: "In the US, a political action committee, or PAC, is the name commonly given to a private group, regardless of size, organized to elect or defeat government officials or to promote legislation." See http://en.wikipedia.org/wiki/Political_action_committee
ConstituentPAC represents voters to their Representatives in the U.S. Congress. (See http://constituentpac.com)
Congressional aides say having contact with constituents, or anyone who speaks to them on issues, makes the issue more immediate and allows them to connect the issue with people. By having someone speak with them on an issue, Congressional aides now have a relationship with someone who can keep them up-to-date on developments related to the issue, and the aides get feedback about how the work done on Capitol Hill is affecting people.
While a consituent of a Congressman can visit his or her local Congressional ofices in a Congressional District, in reality most of the aides who make things happen are usually in the Washington DC Congressional office of a Congressman. Thus, by joining ConstituentPAC.com (it is free to join), a voter gets a lobbyist who presents their issues directly to the Congress on Capitol Hill.