Doubled pawns is when two pawns are on the same file (Remember files go up and down). Doubled pawns are a bad thing to have in chess, are they are not nearly as good as having two pawns side to side. One way to determine if a piece is powerful is by looking at the amount of squares it can move to.
In the example above, the black pawns on e6 and e7 can only move to one square: e5. On the other side of the baord we see that the white pawns can move from f2 to f3 and f4 and from g2 to g3 and g4. 4 moves compared to one; its easy to see why whites pawns are better positioned.