That's a real classic about pawn breakthroughs. This is the most basic example of how you can sometimes even force a breakthrough when it seems there is none.
Black can force the draw with 1... b6 (not 1... a6? 2. c6! or 1... c6? 2. a6 and white wins). Then after 2. cxb6 he recaptures 2... cxb6, or if 2. axb6 then 2... axb6. It's also good to notice that black can't win after 2. axb6 axb6 3. cxb6 cxb6, since after 4. Kg2 Kg6 5. Kf3 Kf5 6. Ke3 Ke5 7. Kd3 Kd5 8. Kc3 Kc5 9. Kb2 white can always draw by playing Kb3 only when black has taken on b5.
A classical example of pawn endings
White to move and win:
A final note: if black starts, then he can force the draw! Find out how