The "hedge around the law" or "fence around the law" is how some people refer to the safety margin that some people put around rules in order to make really sure that they do not break them. For example, the Torah commands the Jewish people not to boil a young goat in its own mother's milk. But what about its grandmother or its cousin? And how do you know if two animals are related? And what about just a cream sauce or cheese? So before long, the rule became that meat and dairy products weren't consumed together, and kosher homes even kept separate dishes for dairy products. There was a hedge or fence put around the law. It's easy to think of this hedge as keeping the law safe, or even keeping us from trespassing on the wrong side of the law.
But as much as I can understand how you might get from Point A to Point B above, I think we need to look at another angle as well. It's not that we were meant to devote our lives to putting a hedge around the law; in fact, the law exists largely to put a hedge around us and our lives. When we consider the Torah or Jesus' teachings, most of God's laws are ethical and community-building laws. Rather than the law being the garden that we need to protect with the hedge, our lives and communities are the garden to protect with the hedge. The law keeps us from being uprooted, trampled, or broken. It makes us more peaceful, happier. It makes our lives and cultures flourish in a way they never could without the protection it affords.
It is a shield for us against many of the troubles of life. We should no more go into the world without God's law than we should walk into a battle unarmed and unshielded.