Mirroring is taking the same posture as the person you are talking with.
If people are involved in conversation and are really in touch with each other they often unconsciously copy the posture and gestures of the one they are speaking with.
You may be able to notice this if you are on a party and look around. If two people have an intimate conversation they may stand or sit in the same way, keeping their legs in the same position, a glass in the same hand, even talking in the same rhythm.
Mirroring may be used deliberately as a flirtatious technique. You observe the way the other person stands or sits and after a while assume the same position. Of course this has to be done subtly! Don’t do it too quickly (wait at least 2-40 seconds) and don’t overdo it. It has to look natural. If you do it well the other person may get the impression to be in touch with you. It’s a simple but powerful way to built rapport as long as the target remains unaware of your intentions.