It means that once you have established a habit, it is significantly easier to to maintain or resume it than it is to change it. The water of habit hollows out a channel, like the formation of a river. And since water follows the path of least resistance, it is most likely to continue to use the existing channel than to try to form an entirely new channel. Thus when you have input (water) that triggers a response in you, it is most likely for it to flow down the same path that it has before, which is your established habit.
This is why it's very difficult to break old habits and form new ones, because you must force the "water" down a different path rather than the familiar channel it has always taken before. Establishing new habits takes time, just as water takes time to carve out a new channel.
Habit formation requires consistent commitment
Image showing the changing course of a river over time