Articulation refers to joints, hinges or any other type of construction that allows a figure movement. Referring to a figure as 'articulated' simply means there's some way to move one or more parts of a figure. 'Much articulation' on a figure then speaks of the ability of the figure to be moved around on many places (points).
Naturally a more-articulated figure will contain more parts and have a more complicated construction than a figure with less articulation.
With the Mattel 6-inch line, Mattel began with simply 9 points of articulation (9POA) but soon became developed into figures with more complicated articulation as the line expanded into DC Superheroes and later DC Universe.
With Justice League, Mattel forayed into more articulation after the initial success of their 5POA figures. When the line began to include large accessories to improve playability, the figures themselves were 'upgraded' with four more points of articulation. When Justice League became rebranded into Justice League Unlimited, the accessories were dropped but the articulation remained, but only for the six lead characters. Thus droves of new figures with 5POA sculpts began to be released alongside the 9POA leads. As the line once again got rebranded, Mattel, due in part to fan demand, returned the 5POA sculpt as it blended better with the newer figures and remained closer to the character designs.