It sounds like complicated math, but a fan array is really just a block of smaller fans. Fan arrays are often used in place of single, larger fans.
Fan arrays allow for redundancy in the system. All that means is that if one fan goes down, the other fans in the array can just pick up the slack, but continue to function. In a single fan system, a fan failure would mean the system could no longer push air.
Fan arrays are classified by their arrangement. For example, the fan array below is a 2×3 array because there are 2 rows of fans, with each row having 3 fans.