Playing Styles of Fischer and Botvinnik They say Botvinnik had 'scientific' approach to chess and Fischer's style was crystal-clear. 

What does it mean?

Could you please enunciate about the styles of these masters in a more lucid way?
Botvinnik believed in peer review.  He would write up annotations for his games, and publish them hoping for feedback from other players.  He also recommended to his students that they annotate their losses, and also look for mistakes in the games they won.  These elements are all part of his 'scientific' approach.

Fischer always kept the endgame in mind, preferred bishops over knights, grabbed pawns if he thought he could get away with it.  Mikhail Tal would wait until the time control drew near, and then suddenly start to complicate the position, trying to induce his opponent into blundering.  Fischer was much more straightforward, playing the board more than the man, not indulging in psychological ploys as much (at the board anyway).  After the game was over, his opponent would look at Fischer's moves, and think they looked obvious in retrospect.  These are the factors that make his style 'crystal clear.'