In Taoist terms, being like a valley is to place yourself humbly beneath others around you. By doing so, even the persona of water--arguably the most powerful and adaptable force on earth--will be directed to your feet by the passive use of gravity. It is a great metaphor in terms of slaying the ego.
In terms of physical application, the valley is symbolic of letting one's opponent come to you. Stillness overcoming force. Gravity, as all martial artists realize, is a powerful tool, and is the root of many a valuable technique.
In fact, it reminds me of my sensei who I've often heard teaching students, when sparring with a taller opponent, to bend down even farther, and slowly lure the opponent into lowering him/herself to reach you. Then when you feel ready, the trick is to pop back up and execute your attack.
This is from the same teacher who insists, somewhat in jest, that the most balanced position the human being can find themselves in is lying flat on his/her back.
Likely some brazilian jiu-jitsu guys might appreciate this comment.