BiS is just the gear in a particular tier that provides the most of whatever stat a particular job desires. For SAM: DH=SkS>Det>Crit.

So lets just take ilvl 320 gear for example.
1 - http://na.finalfantasyxiv.com/lodest...3ef8e8/?patch=
2 - http://na.finalfantasyxiv.com/lodest...6f425b/?patch=

If you wanted to prioritize Direct Hit you would use piece #2 as it has more DH then you could meld into #1. If you wanted to prioritize SkS you would use piece # 1 as it has more than you could meld into piece #2.