Somewhere someone, Setherick I think, explains the math behind how the sliders work. I can't find it otherwise I would link it so you could read it. But, basically the slider number from 0 to 100 means how much that skill "weights" into determining the CUR/FUT value for a player.
Everyone has their own weights so we all see players with different rating values unlike Madden where everyone would see the same ratings for players. So the lower the slider number is, the less that skill rating matters towards determining your overall rating value for a player and vice versa with higher numbers. Generally, for the skills you feel are most important you want to rate them much higher than other skills that you don't feel are as important.
I have posted a set of weights on the forums that I think pretty decent for basic player evaluation if you are new and looking for a set of weights to use.