WHAT DOES A4 MEAN?

Papers manufactured and sold outside of north america are based on
1square meter.The ISO system of paper sizes applies to all grades
of papers and in each grade, each sheet is twice the size of the
next smaller sheet and half the size of the next larger.
The starting point of ISO sizes is the A0 sheet
(which measures 841 x 1189mm) and has an area of 1 square meter.
Half of it is A1, again half of A1 is A2…similarly A4 is 4Th in the row.

The A sheet is used for general printed matter.