These are phrases to which a reaction has become almost instinctive. They vary from one individual to the next, but may include the following: Exclamations such as: "Hello" and other informal greetings; "Hi", "Wotchya", etc. Commands such as "Go away" (and its vulgar counterparts), "Don't do that!" Questions such as: "How are you?" , "Who are you?" , "What's your name?" , "Fancy a beer?" and "Have you got a light?"
Common phrases reduce much ambiguity in communication. For example, without a knowledge of the common phrase "Have you got a light?" Implying a request for a source of ignition, probably for a cigarette, it would be parsed. In parsing this phrase its whole meaning will change. Probably to an interrogation of whether "you" have a source of light energy.
The way in which we, as humans, react to "common phrases" is still debated by psychologists. I believe that slot-and-frame parsing is the answer. It may not be, but it certainly provides a means to model the response to common phrases within a computer program.