It refers to the least significant digit on the meter within that range. As an example, when measuring 120 volts ac on this 6000 count meter. To measure 120 volts ac, the meter will have to be on the 600.0 ac voltage range. The resolution (least significant digit) then is 0.1 volt.
Using the accuracy specification in the title, +/-(1% of reading + 3 counts). Here is how we would determine the accuracy:
+/- (120 V X 1% = 1.2 V + 3 digits) = (1.2 V + 0.3 v) = +/- 1.5 V