What is this?
Matching, Mirroring and Pacing are the conscious acts of making ourselves move, sound and appear similar to the person with whom we are communicating.
Why is this important?
These are basic functions of building likability and congruence with others - gaining rapport. They are based on the fact that we tend to like others who are like us - which increases our influence with the other person and tends to open their mind to us.
Matching the other person's use of terminology, their style of dress, eye contact etc all help to make others feel like they know us to an extent - that we share something in common. Similarly, sitting or standing in a similar position, responding in like manner to their movements (mirroring) and speaking with the same pace and intensity all tend to make someone more comfortable with us.
These skills are most valuable when they have been developed into habits because during development others often see us "copying" what they are doing and saying. We are flattered when children imitate us, but not always when other adults do so. Research indicates people are less receptive to us when they are "on" to our efforts to match, mirror and pace them.
When these skills are used without thought, (and detection) research indicates significant gains in likability and receptiveness. When we make movements which are then followed by the other person, we have significant rapport with them.