The word "curb" has a perfectly clear physical meaning, especially when it is used as a noun. "I just ran over the curb" means "I just ran over the strip of raised asphalt on the edge of the road and the limit of my yard." The experience likely gave the speaker of that sentence an unpleasant bumping sensation, increased maintenance cost for his car and at least one tire, and should deter him from repeating whatever turn or inattention caused the run-in. That, at least, is a key reason why the curb is there. Of course "curb" can also be used less physically. Perhaps one might say metaphorically, but I'm not sure "metaphor" is really the right word. "Republicans elected to Congress in 1946 were devoted to placing new curbs on the power of labor unions." There is also the verb use, "please curb your enthusiasm for this diner -- the food here is going to kill you." A recent television show was built around that...