A polyatomic ion is a group of elements that occur together commonly as an ion and have a net electrical charge.  The term net charge means the sum total of the charges of the individual elements.  We sum these charges together to get the total, or net, charge since the polyatomic ion behaves as a singular unit.  One example is the hydroxide polyatomic ion (OH-).  It consists of an oxygen (-2 charge) and a hydrogen (+1 charge).  The sum total of these charges is -2 + 1 = -1.  So the net charge for the hydroxide ion is -1.  The carbonate ion is CO3(-2).  The oxygens each have a -2 charge (for a total of -6).  The carbon has a +4 charge.  This sums up to a net charge of 4-6 = -2.