Grammage or grams per square meter is the weight of a sheet of paper. While this is the usual way paper is described, printers and copiers can’t measure it: what matters to them is the thickness of a sheet (measured in microns). A good quality sheet can be thicker but have the same weight as a lower quality sheet, which means it’s possible for a 70 g/m2 sheet to be thicker than an 80 g/m2 one.
One good way of checking thickness is to compare two reams of paper side by side and see which one is higher: that’s the thicker one.
Sometimes a printer will struggle to cope with a very flimsy 80 g/m2 sheet, but will quite happily print a 70 g/m2 one. Moreover, there are ecological benefits to using lower grammage papers: lower use of raw materials (for example, if you compare 70g/m2 to 80g/m2, that’s a saving of 10/80, or 12.5%) and a reduction in waste generated.