Here is a quick and concise guide to understand GD&T basics.
"An ideal assembly design comprises of parts which fit each other right….right at the first time! In such a case, the designer is always sure of the assigned clearances and tightness. But in cases where a large number of these parts need replication, conveying the right size and shape (i.e. the nominal dimension of the surface of the part) to the manufacturer is almost impossible. ..."
Read more at https://www.engineersedge.com/gdt.htm