Your skill set encompasses the skills involved in the game. Basically everything except for your physical attributes (speed, strength, fitness etc.). When a coach says for a flanker or a lock to improve their skill set, they generally mean that the player should focus on improving their ball handling (catching, passing) and their rucking, scrummaging and lineout techniques. Hope I've helped.
I have been similiar things like this by my coach and what he means is for you to work on ball handling (good safe hand)and their rucking(robbing ball, seecuring ball), scrummaging(positioning and being quick off the back of the scrum if ur a flanker, particuraly an openside) and lineout techniques(juming,timeing catching). Hope i helped... Go to youtube and look at some videos... there helpful
Skill set is like speed and strength. For a sport like baseball it could be hitting and fielding as well.