what is this grand mastery and speciliztion i hear everybody talking about

As per AD&D rules, pure fighters (and only fighters) with any kit can put up five proficiency slots into a weapon. The levels being: proficient, specialist, master, high master, and finally grand master. With each slot hit roll, damage, and sometimes # of attacks with the chosen weapon increase. When you go from high master to grand master you also get a 1 point boost to speed factor. In BG2 terms this means a grand master specialiist has +2hit/+3dmg, 3att/2rds, and a +1 sf in addition to other abilities conferred on the fighter by his class and /or kit.