Lets say I am looking at SU(5) and I draw a (young taleux) box and I say thats the 5 rep of SU(5). Then I draw 2 boxes stacked on top and I say thats the 10 rep of SU(5)... What does this mean!!!? I know how to calculate the dimension of a young tableux but I don't know what it means in terms of physics and particles. Can anyone shed some light on this for me??