Rapport is a term used to describe, in common terms, that two or more people feel in sync or on the same wavelength because they feel alike.
It stems from an old French verb rapporter which means literally to carry something back; and in the sense of how people relate to each other means that what one person sends out the other sends back, for example they may realise that they share similar values, beliefs, knowledge, or behaviours around sports or politics.
There are a number of techniques that are supposed to be beneficial in building rapport such as: matching your body language (i.e., posture, gesture, etc.); maintaining eye contact; and matching breathing rhythm. Some of these techniques are exploited in NLP (neuro-linguistic programming).